cxgbe(4): Do not ignore the return value of ifmedia_ioctl.

This ensures that the driver reports an error instead of failing
silently when an invalid media is requested.

Reported by: Suhas Lokesha @ Chelsio
Sponsored by: Chelsio Communications

(cherry picked from commit cdd7fe04cb803d49a2fb6389be70f4378ae3ca3b)