scsi: qla2xxx: Change the return type of qla24xx_read_flash_data()

This change makes it easier to detect qla24xx_read_flash_data() failures
and also to handle such failures. This change does not modify the behavior
of the driver since all callers ignore the qla24xx_read_flash_data()
return value.

Cc: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Himanshu Madhani <hmadhani@marvell.com>
Reviewed-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>