staging: rtl8188eu: change return type of Hal_GetChnlGroup88E()

After the removal of code valid only for 5 GHz the function
Hal_GetChnlGroup88E returns always true.

Change the return type to void and remove the variable bIn24G.

Remove the tests for the return value and the variable bIn24G
from the only user Hal_ReadTxPowerInfo88E().

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