msm_serial_hs : handle uart_flush_buffer

Serial core allocates circular buffer for uart tx transfer.
Circular buffer head and tail is updated in serial core and
circular buffer tail is updated in uart driver.While uart
transfer is happening, uart client bluetooth hci_ldisc,calls
uart_flush_buffer in serial core and sets circular buffer
head and tail to zero.As uart driver does not know when
uart_flush_buffer is called in serial core by uart client,it
updates circular buffer tail for the previous ADM Tx completion.
This leads to queueing Tx command to ADM although no data is
there in circular buffer.Because of this,uart client is not
functional.

Hence adding msm_hs_flush_buffer api which notifies to uart driver
that uart_flush_buffer is called in serial core by uart client and
set tty_flush_receive flag to true.In uart interrupt handler ,do
not update circular buffer tail if uart_flush_buffer is already
called in serial core by uart client else update circular buffer
tail on ADM Tx completion.

(cherry picked from commit ce39410ad3db5b9fb446bf5da126585d58fa872a)

Change-Id: I9bce30da218f42739c175d9b3c283ae39b6b5c89
CRs-Fixed: 419054
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>