msm_serial_hs_lite: Check SR[TXEMT] in wait_for_xmitr

Use SR[TXEMT] in addition to ISR[TX_READY] when determining
whether new data can be transmitted to work around a
condition where ISR[TX_READY] may not always give an
accurate indication of the state of the queue.

Change-Id: Ia3a590a485647a005c7c019660f11566208c2667
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>