EMAC-FVP: Fix Ethernet controller instance init

The pointer member `data` of the `gs_eth_device` object was used before being initialised.
Create an instance to assign to the ` data` member in the static initialisation.
Also add check for members before use in native driver.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>