igc: Don't start in promiscious mode by default

The igc driver always sets the promiscious mode during initialization,
ignoring what is set in ifp.
Fix this by checking the interface flags and setting mode appropriately.

Reviewed by: grehan
Obtained from: Semihalf
Sponsored by: Stormshield
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36591

(cherry picked from commit 55760984d56bb1a4c6bc8040ae022353667258e2)