if_awg: Skip emac reset if configured for internal PHY

On the OrangePi One at least, emac reset when an ethernet cable is not
plugged in seems to break ethernet. Soft reset will fail, even with
increasing the delay and retries to wait for up to 20 seconds. This can be
reproduced across at least two different OrangePi One's by simply leaving
ethernet cable unplugged when awg attaches. Whether it's plugged in or not
through u-boot process makes no difference.

Skipping the reset in this configuration doesn't seem to cause any problems,
tried across many many reboots with and without ethernet cable plugged in.

Tested on: OrangePi One
Tested on: Other boards (manu)
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D13974