sh_eth: fix *enum* RPADIR_BIT

The *enum*  RPADIR_BIT  was declared in the commit 86a74ff2 ("net:
sh_eth: add support for Renesas SuperH Ethernet") adding SH771x support,
however the SH771x manual doesn't have the RPADIR register described and,
moreover, tells why the padding insertion must not be used. The newer SoC
manuals do have RPADIR documented, though with somewhat different layout --
update the *enum* according to these manuals...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>