serial: msm_geni_serial: Balance vote for zero size transfer

In a rare case if client sends 0 size data for TX transfer, driver
keeps vote and tries to serve TX transfer. But it returns from middle
and skips unvoting the vote kept, which results in imbalance of TX
path vote.

Note that scenario is very unlikely but in some cases customer will
have negative test case resulting in above scenario.

Change-Id: Ie4724ff40eec2beb67c113a6193377d4166a2b8b
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>