staging/xgifb: Replace XGI340_CR6B table with simple if/else

The XGI340_CR6B lookup table consists of the entries
{0xaa, 0xaa, 0xaa, 0xaa} for an index <= 2 and
{0x00, 0x00, 0x00, 0x00} for all other indices.

The only user XGINew_SetDRAMDefaultRegister340 loops over these 4 values
of a line with a for loop and since all entries are the same for each line
we can simply replace the whole lookup table with a simple if/else assignment.

Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>