amd8111e: don't call napi_enable if configured w/o NAPI

The amd8111e network driver was broken by
bea3348e, which makes the driver
call napi_enable() and napi_disable() even if the driver had been
configured without CONFIG_AMD8111E_NAPI, and thus
netif_napi_add() had not been called on initialization.
This triggers a BUG in napi_enable().

This patch fixes the problem. Please apply.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Signed-off-by: Jeff Garzik <jeff@garzik.org>