IPoIB: Fix hang in napi_disable() if P_Key is never found

After commit fe25c561

 ("IPoIB: Don't enable NAPI when it's already
enabled"), if an interface is brought up but the corresponding P_Key
never appears, then ipoib_stop() will hang in napi_disable(), because
ipoib_open() returns before it does napi_enable().

Fix this by changing ipoib_open() to call napi_enable() even if the
P_Key isn't present.

Reported-by: Yossi Etigin <yosefe@Voltaire.COM>
Signed-off-by: Roland Dreier <rolandd@cisco.com>