[SCSI] qla2xxx: Correct SRB usage-after-completion/free issues.

The driver is incorrectly assuming that the 'sp' reference held
in qla2[x00|4xx]_abort_command() is valid after the mailbox
command is issued to abort the exchange.  It is *not*, as the
command may be completed during interrupt context before control
is returned to the mailbox caller.

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