it821x.c: use dev->revision instead of pci_read_config_byte

Minor cleanup.

Signed-off-by: Brandon Philips <bphilips@suse.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Shawn Lin <shawn@dmp.com.tw>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>