[SCSI] qla2xxx: Rework MSI-X handlers.

Since MSI-X vectors do not require a clearing "handshake" from
the system perspective, and the registered handler will not be
called more than once for one occurrence of receipt of a vector,
there is no requirement to flush the risc register write clearing
the interrupt condition in the risc. Also, since the msi-x
registered handlers are optimised for a particular vector, it is
preferable to handle the one vector received per invocation of
the handler.

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