net: ec_bhf: Add runtime dependencies

The ec_bhf driver is specific to the Beckhoff CX embedded PC series.
These are based on Intel x86 CPU. So we can add a dependency on
X86, with COMPILE_TEST as an alternative to still allow for broader
build-testing.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Darek Marcinkiewicz <reksio@newterm.pl>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>