mux: mmio: Zero the allocated memory

Zero the allocated memory in probe() for fields and hardware states
because:

1. The "hardware_states" array is not initialized in the probe, thus
   starting the device with uninitialized memory.  This not a bug,
   because pointed memory will be assigned in suspend callback, however
   it is a discouraged coding practice.
   The "fields" array is initialized shortly further in the probe().

2. Linux kernel convention for safer code encourages using zeroed
   allocations, as expressed in memory-allocation.rst document.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260317152029.274829-2-krzysztof.kozlowski@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>