net: bcmgenet: Workaround for Pi 4B network issue

Some combinations of Pi 4Bs and Ethernet switches don't reliably get a
DCHP-assigned IP address, leaving the unit with a self=assigned 169.254
address.

Forcing renegotiation has been found to be an effective workaround, so
add an automatic renegotiation after the link comes up for the first
time; enable it with genet.force_reneg=y - by default it is disabled.

See: https://github.com/raspberrypi/linux/issues/3108



Signed-off-by: Phil Elwell <phil@raspberrypi.org>