net: turn name len check into an assert

net: turn name len check into an assert

The new sanity check introduces a printf warning for some systems:
	eth.c:233: warning: format '%zu' expects type 'size_t', but argument 3 has type 'int'

Rather than tweak the format string, use the new assert() helper instead.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>