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

commit fec62c36 upstream.

Most smartarrays tolerate it, but a few new ones don't.
Without this change some newer Smart Arrays will lock up
and i/o will grind to a halt.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>