[SCSI] qla2xxx: Correct execution-throttle setting for ISP24xx.

Similarly to other ISPs, set execution throttle to maximum
allowed value since 'throttling' is done on a per-lun basis
via queue-depth.

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