driver: eth: Fix phy-connection-type setting on RA8 ethernet

This commit fix the pin function configuration for mii and rmii
setting of Renesas RA8 ethernet driver
Correct pin function setting for mii is
PFENET = (uint8_t)(0x1 << R_PMISC_PFENET_PHYMODE0_Pos);
and rmii is:
R_PMISC->PFENET = (uint8_t)(0x0 << R_PMISC_PFENET_PHYMODE0_Pos);
Change code to intialize ping in driver init function, change
method of getting and checking via enum index of device tree.
Add build assert for invalid phy type.

Signed-off-by: Duy Nguyen <duy.nguyen.xa@renesas.com>