ext2fs: fix a check for negative block numbers.

The previous change accidentally left the substraction we
were trying to avoid in case that i_blocks could become
negative.

Reported by:	bde
MFC after:	4 days
Fix a typo.
Always update the hw.uart.console hint anytime a change is made to the
comconsole setup.  Previously the hint would be set when if you set a
custom port, but it would not be updated if you later set a custom speed.

Also, leave the hw.uart.console hint mutable so it can be overridden or
unset by the user if needed.

Reviewed by:	kib (earlier version)
MFC after:	1 week