staging: rtl8188eu: remove unused rtw_hal_write_rfreg

rtw_hal_write_rfreg() is never referenced, so remove it.

It used to be the only place where the write_rfreg callback was
called, so get rid of the function pointer as well.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Greg Kroah-Hartman <gregkh@linux.com>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>