drivers/ethernet: Update default GPIO pin for the ENC28J60 module

All sample applicatons in Zephyr, using the ENC28J60 driver, set
the ETH_ENC28J60_0_GPIO_PIN Kconfig variable to 19.
However, in the Kconfig.enc28j60 file this variable is set to 24.
That default value, 24, was used only during the first iterations
of this driver and never used again.

In this patch, we set the Kconfig variable to 19 and simplify
project configuration files by removing one line.

Change-Id: I3d5fd9da04a3f10845d2a409de56f5b9c235e995
Signed-off-by: Flavio Santes <flavio.santes@intel.com>