[SCSI] qla2xxx: Don't leak SG-DMA mappings while aborting commands.

Original code inadvertently cleared an SRB's 'flags' while
aborting; causing a follow-on scsi_dma_unmap() to be potentially
missed.

Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>