asmc: print the last field of the SMC revision correctly

[seuros]

Summary

Use be16dec() at the correct offset instead of an unaligned,
off-by-two-bytes read through a cast pointer.

MFC after: 3 days

[ngie]

sys/dev/asmc/asmc.c

471

Which model did this get incorrectly printed out on?
Just in case others are wondering, I got this logic original from OpenBSD: https://github.com/openbsd/src/blob/5238f50f352c1b3c3a1bd96feb049609db8e5144/sys/dev/acpi/asmc.c#L301-L302 .

[seuros]

sys/dev/asmc/asmc.c

471

Not model-specific

I compared with MacOs. About This Mac -> System Report -> Hardware X.YZfN

The extra 2 char is just garbage as it differ between machines (same model)