Bluetooth: AT: Fix reset the state during error

1. Reset the state when error occurs
2. This patch avoids checking for the AT_STATE_UNSOLICITED_CMD state
as the loop exits with possix error. So check for at->state
AT_STATE_START is sufficient for loop termination.
3. Fix different sign warning.

Change-Id: I80a1ca582112f1783690ac8a4125036bb706705f
Signed-off-by: Sathish Narasimman <sathish.narasimman@intel.com>