staging/xgifb: Simplify XGI_GetRatePtrCRT2

Since the smaller LCDRefreshIndex is contained identically in LCDARefreshIndex
we can simply use LCDARefreshIndex and skip the if/else.

Since LCDARefreshIndex is only used readonly and contains only small
unsigned values we also change its declaration to const u8.

In order to prevent an out-of-bounds access I changed the mask from 0x0F
to 0x07 and added a dummy value.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>