rt2800: move rf init calibration code

Add separate function for rf init calibration code and use it
on all init rf subroutines.

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>