msm_serial_hs: Handle Tx request while Clock OFF

If client initiates a Tx operation when UART clocks are
Off, the driver should not attempt to send data, as this
would result in interacting with unclocked hardware. Add
support to detect this condition and avoid writing to an
unclocked device.
Add a null check after memory allocation.

CRs-Fixed: 629870
Change-Id: I001516e731e73881a29d768d9fb1bf759f3419b9
Signed-off-by: Dilip Kota <c_dkota@codeaurora.org>
Signed-off-by: Dilip Kota <c_dkota@codeaurora.org>