smsc9420: Use netif_<level>

Use a more standard logging style.

Convert smsc_<level> macros to use netif_<level>.
Remove unused #define PFX
Add pr_fmt and neaten pr_<level> uses.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>