drivers: ethernet: enc28j60: Add DT property to set Rx filter

Byte value written to the device's
ERXFCON: ETHERNET RECEIVE FILTER CONTROL REGISTER
Sets the devices receive packet filter, optional. If not set
in device tree previous hard coded value`0xA3` is used.
Uni, multi and broadcast packets with valid CRC are accepted.

Signed-off-by: Dean Sellers <dsellers@evos.com.au>