mlxsw: spectrum: Extract work-scheduling into a new function

The boilerplate to schedule NETEVENT_IPV4_MPATH_HASH_UPDATE and
NETEVENT_IPV6_MPATH_HASH_UPDATE handling is almost equivalent to that of
NETEVENT_IPV4_FWD_UPDATE_PRIORITY_UPDATE that's coming in the next
patch. The only difference is which actual worker function should be
called. Extract this boilerplate into a named function in order to allow
reuse.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>