Changing the encoding of a ucc keyboard doesn't make sense as only one

Changing the encoding of a ucc keyboard doesn't make sense as only one
encoding is supported. Instead, silently ignore such requests. Gets rid
of the following warning emitted by kbd(8) while booting with a ucc
keyboard attached and /etc/kbdtype being present:

	kbd: unsupported encoding uk on /dev/wskbd2

I ended up repurposing KB_MACHDEP as is became unused back in 2008. Note
that running a kernel with this commit applied requires kbd and
wsconsctl to be recompiled in order to show correct encodings.

Problem reported by landry@ and ok deraadt@