sfxge: retry VF vAdaptor allocation if it fails because of no EVB port yet

After an MC reboot, a VF driver may reset before the PF driver has
finished bringing everything back up. This includes the VFs EVB port.
MC_CMD_VADAPTOR_ALLOC is the first MCDI call after an MC reboot to
require the EVB port, so if it fails with MC_CMD_ERR_NO_EVB_PORT,
retry the command a few times after waiting a while.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4333