sh_eth: rename ARSTR register bit

The Renesas RZ/A1H manual names the software reset bit in the software reset
register (ARSTR) ARST which makes a bit more sense than the ARSTR_ARSTR name
used now  by the driver -- rename the latter to ARSTR_ARST.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>