wil6210: make sure DR bit is read before rest of the status message

Due to compiler optimization, it's possible that dr_bit (descriptor
ready) is read last from the status message.
Due to race condition between HW writing the status message and
driver reading it, other fields that were read earlier (before dr_bit)
could have invalid values.

Fix this by explicitly reading the dr_bit first and then using rmb
before reading the rest of the status message.

Change-Id: I8107d4ee8acafb2922ff86cd951540f97500de95
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>