[SUNGEM]: Fix NAPI regression with reset work

sungem's gem_reset_task() will unconditionally try to disable NAPI even
when it's called while the interface is not operating and hence the NAPI
struct isn't enabled. Make napi_disable() depend on gp->running.

Also removes a superfluous test of gp->running in the same function.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>