[SCSI] mpt2sas: Fix possible integer truncation of cpu_count

When computing reply_queue_count (the number of MSI-X vectors to use),
the driver does

	ioc->reply_queue_count = min_t(u8, ioc->cpu_count,
	    ioc->msix_vector_count);

However, on a big machine, ioc->cpu_count could be outside the range
that fits in a u8; eg a system with 256 CPUs will end up
reply_queue_count set to 0.

Fix this by calculating the minimum as ints and then letting the
assignment to reply_queue_count handle integer demotion.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Acked-by: "Nandigama, Nagalakshmi" <Nagalakshmi.Nandigama@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>