davinci_emac: fix kernel oops when changing MAC address while interface is down

Check that network interface is running before changing its MAC address.
Otherwise, rxch is accessed when it's NULL - causing a kernel oops.
Moreover, check that the new MAC address is valid.

Signed-off-by: Pablo Bitton <pablo.bitton@gmail.com>
Signed-off-by: Chaithrika U S <chaithrika@ti.com>
Tested-by: Chaithrika U S <chaithrika@ti.com>
[tested on DM6467 EVM]
Signed-off-by: David S. Miller <davem@davemloft.net>