rtl8187: Fix driver to return TX retry info for RTL8187L

Current code for the RTL8187 is not returning valid retry information, thus the
rate-setting mechanism is not functioning. As a further complication, this info
is only obtained by reading a register, which cannot be read while in interrupt
context.

This patch implements the TX status return to mac80211 through the use of a
work queue.

One additional problem is that the driver currently enables the rate fallback
mechanism of the device, which conflicts with the mac80211 rate-setting
algorithm. This version of the patch disables rate fallback.

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