net: qrtr: Rearrange tx wait and protect critical section

Check tx wait conditions before entering wait. This helps threads with
pending signals finish their tx's instead of returning ERESTARTSYS.

Make checking the flow count and subsequent waiter operations an atomic
operation. The waiter list and flow count are modified on the tx and rx
threads which opens it to race conditions.

Change-Id: I7cba0e70d8e1760ed2d8cece6404b995721bdcc9
Signed-off-by: Chris Lew <clew@codeaurora.org>