sh_eth: use correct name for ECMR_MPDE bit

This bit was wrongly named due to a typo, Sergei checked the SH7734/63
manuals and this bit should be named MPDE.

Suggested-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: David S. Miller <davem@davemloft.net>