staging/xgifb: Remove unused variable

After the patch
'staging/xgifb: Don't write the same values x times'
the local variable i is unused, which leads to the following warning:

driverrs/staging/xgifb/vb_init.c: In function
‘XGINew_SetDRAMDefaultRegister340’:
drivers/staging/xgifb/vb_init.c:433:43: warning: unused variable ‘i’
[-Wunused-variable]

This patch fixes this

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