drivers: net: Mark functions as static in stmmac_platform.c

This patch marks the function stmmac_pltfr_freeze() and
stmmac_pltfr_restore() in stmmac_platform.c as static because they are
not used outside this file.

Thus, it also removes the following warnings in
ethernet/stmicro/stmmac/stmmac_platform.c:

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c:222:5: warning: no previous prototype for ‘stmmac_pltfr_freeze’ [-Wmissing-prototypes]
drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c:236:5: warning: no previous prototype for ‘stmmac_pltfr_restore’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: David S. Miller <davem@davemloft.net>