[SCSI] qla2xxx: Free DMA memory only if it is allocated in vendor specific BSG command

The qla84xx_mgmt_cmd function supports multiple subcommands.
Some subcommands require DMA memory and some do not.
Therefore check before freeing DMA memory whether it is allocated or not.

Signed-off-by: Harish Zunjarrao <harish.zunjarrao@qlogic.com>
Signed-off-by: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>