net: dsa: Export suspend/resume functions

In preparation for allowing switch drivers to implement system-wide
suspend/resume functions, export dsa_switch_suspend and
dsa_switch_resume() such that these are callable from the appropriate
driver specific suspend/resume functions.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>