serial: msm_geni_serial: Update wakeup byte handling mechanism

In current implementation, after vote_clock_off we check for wakeup
byte in UART Rx buffer. If wakeup byte is present, we send it to BT
Application else drop the rx bytes until wakeup byte 0xFD is received.
The problem with this implementation was if there was out of sync
between Tx and Rx and a packet was received after clock off, it
was checked for wakeup byte which resulted in drop of some rx bytes.

To mitigate above scenario change handling of wakeup byte mechanism as
below:
1. Check the rx packet for wakeup_byte only after runtime suspend
instead of vote clock off.
2. Check only the first byte of RX buffer.
3. If wakeup byte is received send it to BT appplication.
4. Else drop the whole bytes.

Change-Id: I1fea3b99193887d05ee7756df933abd94b6d71d3
Signed-off-by: Prasanna S <quic_prass@quicinc.com>
Signed-off-by: Panicker Harish <quic_pharish@quicinc.com>