eth: Add full-duplex configuration to ENC28J60

The driver originally gets the full-duplex configuration
from the PHCON1 register and PDPXMD bit as stated by the
ENC298J60 specification document section 2.6 "LED Configuration".
This section trust the board to LEDB connection to signal the
full-duplex configuration.

This commit gets the full-duplex configuration from Kconfig
symbols to allow a proper functionality even when the board
is not connecting LEDB apropriately.

Change-Id: I803958409a611e23e2c3e03f40b412f1695947af
Signed-off-by: Juan Manuel Cruz <juan.m.cruz.alcaraz@intel.com>