wil6210: relaxed check for BACK start sequence

Sometimes, due to the race between Rx path and WMI_BA_STATUS_EVENTID WMI event,
few frames may be passed to the stack before reorder buffer allocated.
Then, after BACK establishment, it start getting frames with sequence number ahead of
SSN, and it get interpreted as missing frames. Then, BACK mechanism will wait
for missing frames; data traffic will be stopped. In case of interface configured
for DHCP, this data delay causes DHCP failure.

Relax checking for sequence number; use sequence of 1-st frame handled by the buffer
as SSN for this buffer.

This is work-around, real fix should be done when proper BACK mechanism implemented.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>