sh_eth: consolidate sh_eth_reset()

This driver has sh_eth_reset() function for each SoC and this function is almost
always the same, except for the several a bit different variations for Gigabit
Ethernet.  Consolidate every variation into a single function -- which allows
us to get  rid of some more #ifdef'fery.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
[Sergei: moved the new sh_eth_reset() and sh_eth_is_gether() up to decrease the
patch size, fixed function call continuation lines' indentation, reworded the
changelog, reworded the subject, changing the prefix.]
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>