serial: msm_geni_serial: Fix the issue with PM usage

During system suspend PM_runtime is disabled by PM core for
a device. Currently driver using pm_runtime_enable checks in
PM suspend/resume callbacks and doing manual resume. With this
PM usage count is getting miss-matched in driver and causing
the device to not to enter system suspend state.

Fix this issue by removing pm_runtime_enable checks inside
PM callbacks of driver.

Change-Id: I7032bf28c22eda5b989ee66c452a43ed5b7af75e
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>