extend cb_writel(), exca_writeb(), exca_writel() to do a read[lb]() after
the write[lb]() to avoid possible problem with PCI write posting.

Seems to fix Bug #5061.
Signed-off-by: NDaniel Ritz <daniel.ritz@gmx.ch>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>