At least no in-kernel CardBus-capable PCI driver makes use of the CIS
access functions. Therefore, it seems sensible to remove this unused
code, and cleanup cardbus.c a lot.

CC: Jesse Barnes <jbarnes@virtuousgeek.org>
CC: Linus Torvalds <torvalds@linux-foundation.org>
Tested-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>