[SCSI] qla2xxx: Don't depend on mailbox return values while enabling FCE tracing.

Recent firmwares no longer return the 'number of buffers' in
mailbox6.  The original code may result in a potential panic
during a FW-dump process due to the driver misinterpreting the
size of the allocated buffer.

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