staging: xgifb: vb_setmode: reduce if nesting in XGI_GetVCLK2Ptr()

Eliminate unnecessary nesting levels by rearranging code and conditions.
The resulting code should be still identical.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>