sh_eth: fix dumping ARSTR

ARSTR  is always located at the start of the TSU register region, thus
using add_reg()  instead of add_tsu_reg() in __sh_eth_get_regs() to dump it
causes EDMR or EDSR (depending on the register layout) to be dumped instead
of ARSTR.  Use the correct condition/macro there...

Fixes: 6b4b4fea

 ("sh_eth: Implement ethtool register dump operations")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>