serial: msm_geni_serial: Avoid unclocked access from start_tx

During system suspend/resume testing, when console is in suspended
state and clocks are OFF. Seems UART console used to dump some info on
console and leading to NOC fault.

To fix this issue add defensive check in msm_geni_serial_start_tx() api.

Change-Id: I27edd7814e28946449afe256f72f7cb1c868822e
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>