ibmvscsi: Add and use enums for valid CRQ header values

The PAPR defines four valid header values for the first byte of a CRQ
message. Namely, an unused/empty message (0x00), a valid
command/response entry (0x80), a valid initialization entry (0xC0), and
a valid transport event (0xFF). Further, initialization responses have
two formats namely initialize (0x01) and initialize complete
(0x02). Define these values as enums and use them in the code in
place of their magic number equivalents.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Reported-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Manoj Kumar <manoj@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>