IPoIB: Don't enable NAPI when it's already enabled

If a P_Key is not present when an interface is created, ipoib_open()
will return after doing napi_enable().  ipoib_open() will be called
again from ipoib_pkey_poll() when the P_Key appears, after NAPI has
already been enabled, and try to enable it again. This triggers a
BUG_ON() in napi_enable().

Fix this by moving the call to napi_enable() to after the test for
P_Key presence.

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