rtc: imxdi: remove unused variable

This variable is no longer used and the compiler rightly complains that
it should be removed.

../drivers/rtc/rtc-imxdi.c: In function ‘dryice_rtc_set_alarm’:
../drivers/rtc/rtc-imxdi.c:633:16: warning: unused variable ‘now’ [-Wunused-variable]
  unsigned long now;
                ^~~

Rework to remove the unused variable.

Fixes: 629d488a ("rtc: imxdi: remove unnecessary check")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>