rtl8187b: do not do per packet TX AGC

Clearing the per packet TX AGC for the RTL8187B device appears to
increase its overall TX power. This allows the device to associate and a
connection to be established using APs a little further away.

This is in accordance to what is done for RTL8187L devices and also what
Realtek drivers do.

Tested-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Cc: linux-wireless@vger.kernel.org
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Rogerio Luz Coelho <rogluz.news@gmail.com>
Cc: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Cc: Hin-Tak Leung <hintak.leung@gmail.com>
Cc: seno <senada@t-online.de>
Tested-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Signed-off-by: John W. Linville <linville@tuxdriver.com>