[SCSI] hpsa: fix bad comparison

'!' has higher precedence than '&'.  CFGTBL_ChangeReq is 0x1 so the
original code is equivelent to if (!doorbell_value) {...

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>