wl1271: Clean up TX security sequence number handling

Instead of managing the TX security sequence number as two variables, use
one 64 bit variable. This greatly simplifies the handling of the number.

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