staging: rtl8188eu: change array type to const u8

Change the type of MCS_rate_1R from unsigned char to const u8.

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