serial: msm-geni-serial: Update is_clock_off to atomic variable

Update is_clock_off and wakeup_enabled variable to atomic
to avoid race conditions while accessing it.

Change-Id: I7e792b5938befeeede60f15353be328d20c25758
Signed-off-by: Anvesh Salveru <asalveru@codeaurora.org>
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>