rtl8187: fix 8187B throughput regression

Hin-Tak Leung reported that after the change "rtl8187: add short slot
handling for 8187B" his RTL8187B started to give low throughput on
network transfers. Turns out that the SIFS setting used isn't ok, it
doesn't look to be the real aSIFSTime, using the "magical" 0x22 value
like on other 818x variants as the vendor does too fixes the issue.

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>