igb: fix anoying type mismatch warning on rx/tx queue sizing

When using "min()", the types of both sides should match.  With the cpu
mask changes, the type of num_online_cpus() will now depend on config
options. Use "min_t()" with an explicit type instead.

And make the rx/tx case look the same too, just for sanity.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>