msm_serial_hs: Implement driver functionality to be closer to HW spec

Implement TX callflow to be in accordance with HW spec and make
sure TXEMT clock off logic is correct. On receiver side, issue
force stale interrupt and make sure upper layer get bytes upto
size of RX buffer

CRs-Fixed: 575119
Change-Id: Iee469b0ba489386b571a86897b62eae84ff34ca5
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>