staging: mt7621-pci: Include preferred headers

This fixes the following checkpatch warnings:
Use #include <linux/pci.h> instead of <asm/pci.h>
Use #include <linux/io.h> instead of <asm/io.h>

Signed-off-by: Oliver Hübers <oliver.huebers@web.de>
Signed-off-by: Vanessa Borgmann <nessa.x@web.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>