staging: rtl8188eu: reuse Hal_GetChnlGroup88E()

Use Hal_GetChnlGroup88E() instead of duplicating it's code
in get_rx_power_val_by_reg().

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>