rtc: mrst: remove set but not used variable 'valid'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/rtc/rtc-mrst.c: In function 'mrst_procfs':
drivers/rtc/rtc-mrst.c:264:29: warning:
 variable 'valid' set but not used [-Wunused-but-set-variable]
  unsigned char rtc_control, valid;

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>