mxge(4) should pass unhandled ioctls to ether_ioctl()

Panasas discovered that ioctl(SIOCGLAGGPORT) returns ENOTTY for mxge(4) when
the NIC is not a member of a lagg. This came as a surprise, because the
SIOCGLAGGPORT handler in if_lagg.c only returns ENOENT (if run against the
laggX interface, rather than a physical port) or EINVAL (if run against a
non-member physical port). This behavior was not seen with other drivers,
such as bge(4), igb(4), and cxl(4). When I compared their respective ioctl
handlers, I found that they all called ether_ioctl() for the default (i.e.
unhandled) case; by contrast, mxge(4) only calls ether_ioctl() for two
specific cases, and returns ENOTTY for the default case.

Remove the two cases which explicitly call ether_ioctl(), and let the
default case call it instead. This matches what the vast majority of the NIC
drivers do.

Reviewed by: kmacy
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14381