sh_eth: factor out register bit twiddling code

The  driver has often repeated pattern of reading a register,  AND'ing and/or
OR'ing some bits  and writing  the  value back. Factor the pattern out into
sh_eth_modify() -- this saves  84 bytes of code with ARM gcc 4.7.3.

While at it, update Cogent Embedded's copyright.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>