rtl8187: remove redundant initialization of ARFR

This removes redundant write to Auto Rate Fallback Register on RTL8187B.
The same value was being written twice in the same function. Avoid this
removing the duplicate initialization on rtl8187b_reg_table, and also
add comment for this write (information from Realtek source).

Signed-off-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>