rtl8180: fix wrong parameter in sa2400_rf_set_channel

rtl8180: fix wrong parameter in sa2400_rf_set_channel

The sa2400 RF code needs to invoke sa2400_write_phy_antenna every time the
channel is being switch.

This should be done passing the channel number to that function.
Incorrectly we were passing the same value that is written on the
channel RF register.
This may cause problems when operating on ch 14.

This patch fixes it.

Thanks to Alessandro Di Marco who found this issue!

Signed-off-by: Andrea Merello <andreamrl@tiscali.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>