staging/xgifb: Remove duplicated code from XGINew_DDRSizing340

Since XGINew_SetDRAMSize20Reg now handles both cases we can remove
the code duplication in XGINew_DDRSizing340.

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