[SCSI] qla2xxx: Explicitly set the execution-throttle with recent ISPs.

Firmware semantics changed for 24xx and above ISPs in their
handling of the specified execution-throttle passed during
firmware initialization.  The original codes use of a theoretical
maximum (0xffff, as carried over from earlier ISPs) could in fact
act as a throttle in some circumstances.  Now set the value based
of the firmware's own 'resource' (exchange IOCBs) capabilities.

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