[SCSI] qla2xxx: Use pci_try_set_mwi().

As the "must-check" return-value of pci_set_msi() is never
really checked.

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