[SCSI] qla2xxx: Always serialize mailbox command execution.

Original code would incorrectly bypass serialization if the DPC
thread were performing a big-hammer operation (ISP abort).  This
short circuit, though rare, would subsequently stomp on a
secondary thread's mailbox command execution.  Found during
ISP81XX testing.

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