lan78xx: mark symbols static where possible

We get a few warnings when building kernel with W=1:
drivers/net/usb/lan78xx.c:1182:6: warning: no previous prototype for 'lan78xx_defer_kevent' [-Wmissing-prototypes]
drivers/net/usb/lan78xx.c:1409:5: warning: no previous prototype for 'lan78xx_nway_reset' [-Wmissing-prototypes]
drivers/net/usb/lan78xx.c:2000:5: warning: no previous prototype for 'lan78xx_set_mac_addr' [-Wmissing-prototypes]
....

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
so this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>