[SCSI] isci: improvements in driver unloading routine

This patch fixes scenario where driver removal should be possible
only when driver is in READY state. Also it removes redundant
invocation of routine disabling SCU interrupts - this method is
called somewhere else in driver deinitialization path.

Signed-off-by: Andrzej Jakowski <andrzej.jakowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>