dsa: Rename dsa_resolve_tag_protocol() to _get ready for locking

dsa_resolve_tag_protocol() is used to find the tagging driver needed
by a switch driver. When the tagging drivers become modules, it will
be necassary to take a reference on the module to prevent it being
unloaded. So rename this function to _get() to indicate it has some
locking properties.

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