net: dsa: bcm_sf2: move GPHY enabling to its own function

Move the code that touches the single GPHY register from
bcm_sf2_sw_resume() to a separate function since we will have to
enable/disable the GPHY from different locations, and we want the code
to be self-contained.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>