scsi: pmcraid: fix minor sparse warnings

pmcraid_minor is only used in this one file and should be 'static' as suggested
by sparse:

drivers/scsi/pmcraid.c:80:1: warning: symbol 'pmcraid_minor' was not declared. Should it be static?

In Linux coding style, a literal '0' integer should not be used to represent
a NULL pointer:

drivers/scsi/pmcraid.c:348:29: warning: Using plain integer as NULL pointer
drivers/scsi/pmcraid.c:4824:49: warning: Using plain integer as NULL pointer

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>