staging:r8188eu: refactor rtw_get_cur_max_rate() - remove rtw_hal_get_hwreg() call

rtw_hal_get_hwreg(..., HW_VAR_RF_TYPE,...) always return RF_1T1R value.
Replace the function call with RF_1T1R value
and refactor rtw_get_cur_max_rate().

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>