megaraid_sas: do not process IOCTLs and SCSI commands during driver removal

Do not process any SCSI and IOCTL command further (return them with
appropriate return values to callers), while driver removal is in
progress or PCI shutdown is invoked.

Signed-off-by: Sumit Saxena <sumit.saxena@avagotech.com>
Signed-off-by: Kashyap Desai <kashyap.desai@avagotech.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>