rtc: mpfs: remove 'pending' variable from mpfs_rtc_wakeup_irq_handler()

cppcheck reports
[drivers/rtc/rtc-mpfs.c:219]: (style) Variable 'pending' is assigned a value that is never used.

The fetched CONTROL_REG stored in pending is unused and partially
duplicates the functionality of the later call to mpfs_rtc_clear().  This looks
like leftover development code, so remove pending.

Fixes: 0b31d703 ("rtc: Add driver for Microchip PolarFire SoC")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220627205943.2075043-1-trix@redhat.com