Input: tca8418_keypad - enable overflow mode per datasheet (SCPS215G)

The driver currently sets only the overflow interrupt enable bit
(OVR_FLOW_IEN) in the configuration register, leaving the overflow
mode bit (OVR_FLOW_M) at its default value of 0.

According to the TCA8418 datasheet (SCPS215G, Section 8.6.4.1
"Overflow Errata - Description"), both OVR_FLOW_M (Bit_5) and
OVR_FLOW_IEN (Bit_3) must be set high for the overflow interrupt
to be generated. If only OVR_FLOW_IEN is set, FIFO overflow events
are silently lost without notifying the host.

Fix this by setting OVR_FLOW_M alongside OVR_FLOW_IEN in the
configuration register.

Signed-off-by: Zhian Liang <liangzhan5dev@gmail.com>
Link: https://patch.msgid.link/20260529013900.43854-1-liangzhan5dev@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>