cciss: do not attempt to read from a write-only register

Most smartarrays will tolerate it, but some new ones don't.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Note: this is a regression caused by commit 1ddd5049
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>