usbnet: smsc95xx: Clean up unnecessary BUG_ON() upon register access

smsc95xx_read_reg() and smsc95xx_write_reg() call BUG_ON() if the
struct usbnet pointer passed in is NULL.

The functions have just been amended to dereference the pointer on
entry.  So the kernel now oopses if the pointer is NULL, eliminating
the need for an explicit BUG_ON().

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: David S. Miller <davem@davemloft.net>