sfxge(4): improve PCIe link speed and width check

Perform a more accurate check of whether the PCIe bandwidth is
sufficient for the current/supported port modes.

Give a different warning if there is sufficient bandwidth to achieve
line rate, but the link is not fast enough for optimal latency.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6369