tests/sys/net/if_bridge_test: fix MTU test

"ifconfig gif0 ether" doesn't return any output, so this wasn't
correctly checking the MTU on the gif interface. Remove "ether".

Reviewed by: zlei, kp
Differential Revision: https://reviews.freebsd.org/D51245

(cherry picked from commit e6ef675de261b15ed66d434d054a6e5e69308ce5)