net: Fix build issues with arm-clang

The arm-clang compiler/linker does not optimize away unused function
symbols and thus will error if symbols that are referenced are not
defined.  To fix this add needed ifdef'ry.

Build various network samples that utilize ethernet but don't have
CONFIG_NET_PROMISCUOUS_MODE will get a link error for:

	Error: L6218E: Undefined symbol
	net_mgmt_NET_REQUEST_ETHERNET_SET_PROMISC_MODE
	(referred from ethernet.o).

Fix by adding ifdef protection around promisc code.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>