net: stmmac: make tc_flow_parsers static

The tc_flow_parsers is not used outside of the driver, so
make it static to avoid the following sparse warning:

drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c:516:3: warning: symbol 'tc_flow_parsers' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>