Add another check to com probe

Add another check to com probe

With this bentley@'s Steam Deck no longer attaches these:
com0 at isa0 port 0x3f8/8 irq 4: ns8250, no fifo
com1 at isa0 port 0x2f8/8 irq 3: ns8250, no fifo
com2 at isa0 port 0x3e8/8 irq 5: ns8250, no fifo

From Matthias Drochner in NetBSD rev 1.154
ok kettenis@