serial: msm_geni_serial: Avoid UAF memory access in exit path

In issue case, geni_wake pointer is initialized while UART probe
and as a part of UART exit path wakeup_source_unregister is being
called to free the geni_wake pointer from memory but still caller
function can have the garbage value of geni_wake pointer which is
being accessed in uart_remove_one_port and leading to UAF.

To fix the issue we will make geni_wake pointer value as NULL after
freeing it to from memory.

Change-Id: I213026286bef9774f7a426b9348a7302d3e67ff0
Signed-off-by: Yatish Kumar Singh <quic_yatishku@quicinc.com>