msm_serial_hs: Race Condition during Baud Rate Configuration

Race Condition:
==============
CPU 0:
1.) Rx BAM pipe got disconnected during Baud Rate Cnfg
2.) Rx tasklet got scheduled
3.) Tasklet is issuing Rx command to BAM
4.) BAM API aquired spinlock and accessed Null Pointer
This lead to Null Pointer Access and Crash
CPU 1:
5.) As part of baud rate configuration process CPU 1
called BAM Rx Connect API
6.) BAM API is trying acquire Spinlock
7.) Spinlock is not released by CPU0
This lead to BUG ON

Fix:
===
During Baud Rate Configuration, if Rx command is queued
set the flag for not issuing any Rx command to BAM
Adding rx_cmd_exec variable to avoid command getting
queued twice without callback for the first

Change-Id: I800180cba22b1d0326147a21f175d06e4641f6c4
Signed-off-by: Dilip Kota <c_dkota@codeaurora.org>