iflib: remove assertion that isc_capabilities is nonzero

It's atypical, but not invalid, for a driver to pass no capabilities.

Submitted by: Gerald Aryeetey <aryeeteygerald_rogers.com>
Reviewed by: shurd
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20142