hpsa: fix non-x86 builds

commit 28e13446 "[SCSI] hpsa: enable unit attention reporting"
turns on unit attention notifications, but got the change wrong for
all architectures other than x86, which now store an uninitialized
value into the device register.

Gcc helpfully warns about this:

../drivers/scsi/hpsa.c: In function 'hpsa_set_driver_support_bits':
../drivers/scsi/hpsa.c:6373:17: warning: 'driver_support' is used uninitialized in this function [-Wuninitialized]
  driver_support |= ENABLE_UNIT_ATTN;
                 ^

This moves the #ifdef so only the prefetch-enable is conditional
on x86, not also reading the initial register contents.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 28e13446 "[SCSI] hpsa: enable unit attention reporting"
Cc: stable@vger.kernel.org # v3.14+
Acked-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>