uart: only use MSI on devices that advertise 1 MSI vector

This updates r311987/fb1d9b7f4113d which allowed any number of vectors to be
used. Since we're just attaching one instance, the meaning of more than one
vector is not clear and seems to cause problems. Fall back to old methods for
these cards.

PR: 235016
Submitted by: David Cross
Approved by: re@ (gjb)
(cherry picked from commit 955b6109bb36036e9357006be42dfa89cd7cb0f2)
(cherry picked from commit ef1f2056084202c554d5482af9586a8995e604d5)