sfxge: use if_initbaudrate() to set interface baudrate

It is required to cope with u_long limit for 10 Gbps in a right way.
It is a direct commit to stable/10 since head differs (head does not
have if_initbaudrate(), if_baudrate is simply uint64_t).

Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D2849