bcmgenet: Better coalescing parameter defaults

Set defaults for TX and RX packet coalescing to be equivalent to:

  # ethtool -C eth0 tx-frames 10
  # ethtool -C eth0 rx-usecs 50

This may be something we want to set via DT parameters in the
future.

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