wl1271: Prevent dropping of TX frames in joins

The wl1271 uses a session counter in CMD_JOIN and TX frame descriptors. This
counter is used to determine which frames to drop when the CMD_JOIN is
executed.

The driver executes CMD_JOIN multiple times upon association and sometimes
disassociation, and we don't want any frames to get lost.

Fix this by incrementing the session counter only when leaving idle (not every
CMD_JOIN as before.) Also, remove the TX flush flag from the CMD_JOIN options.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>