net: dsa: bcm_sf2: reset switch prior to initialization

Our boot agent may have left the switch in an certain configuration
state, make sure we issue a software reset prior to configuring the
switch in order to ensure the HW is in a consistent state, in particular
transmit queues and internal buffers.

Fixes: 246d7f77 ("net: dsa: add Broadcom SF2 switch driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>