x86, swiotlb: Simplify pci_swiotlb_detect_override()

At the end of the function, the local variable use_swiotlb has always
the same value as the global variable swiotlb. Hence drop the local
variable completely.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>