stand: eficom: Only set baudrate when it changes

Only set the baudrate when it is different than what the device has
reported. In addition, pass in the args to effect no change to the other
parameters to the serial port. Some EFI firmware gets cranky when you
set them to the same value, so avoid doing so (we likely can remove the
HyperV workaround with this fix, but I kept it in place). Add comments
to the code for why we do this too.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D40010