Fix broken malloc in e6000sw

Malloc should always return something when M_WAITOK flag is used,
but keep this code and change flag to M_NOWAIT as it is under a lock
(allows for possible future change). Free ifnet structure to avoid
memory leak on failure.

Submitted by: Zbigniew Bodek <zbb@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: loos
Differential revision: https://reviews.freebsd.org/D10711