rtl8187: feedback transmitted packets using tx close descriptor for 8187B

Realtek 8187B has a receive command queue to feedback beacon interrupt
and transmitted packet status. Use it to feedback mac80211 about status
of transmitted packets. Unfortunately in the course of testing I found
that the sequence number reported by hardware includes entire sequence
control in a 12 bit only field, so a workaround is done to check only
lowest bits.

Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Tested-by: Hin-Tak Leung <htl10@users.sourceforge.net>
Signed-off-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Signed-off-by: John W. Linville <linville@tuxdriver.com>