iio: adc: ad_sigma_delta: fix clear_pending_event for registerless devices

ad_sigma_delta_clear_pending_event() falls through to the status register
read path for devices with has_registers = false and no rdy_gpiod. For
such devices, ad_sd_read_reg() skips the address byte entirely and clocks
raw MISO bytes with no address phase — making it byte-for-byte identical
to reading conversion data. If a pending conversion result is present,
this partially consumes it and corrupts the data stream for the subsequent
ad_sd_read_reg() call in ad_sigma_delta_single_conversion().

Furthermore, with num_resetclks = 0 on these devices, data_read_len
evaluates to 0. If the clocked byte has bit 7 clear, pending_event is set
and the code attempts memset(data + 2, 0xff, 0 - 1), overflowing to
SIZE_MAX and corrupting the heap.

Fix by returning 0 immediately when neither rdy_gpiod nor has_registers
is set. This is safe for all current registerless devices: ad7191 and
ad7780 (with powerdown GPIO) are reset between conversions by CS
deassertion, so there is no stale result to drain; ad7780 (without
powerdown GPIO) and max11205 are continuously-converting and cycle ~DRDY
at the output data rate regardless of whether the previous result was
read, so the next falling edge fires naturally.

A future registerless device that holds ~DRDY asserted until data is read
would be broken by this early return and would require either
num_resetclks set or a rdy-gpio.

The same heap corruption is reachable on any device with rdy_gpiod set
but num_resetclks = 0: if the GPIO indicates a pending event, the drain
path executes memset(data + 2, 0xff, 0 - 1) regardless of has_registers.
Add an explicit data_read_len == 0 guard after the pending event check;
the stale result is then consumed by the first ad_sd_read_reg() call in
ad_sigma_delta_single_conversion().

Fixes: 132d44dc6966 ("iio: adc: ad_sigma_delta: Check for previous ready signals")
Signed-off-by: Radu Sabau <radu.sabau@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>