bnx2: Fix hang during rmmod bnx2.

The regression is caused by:

commit 4327ba435a56ada13eedf3eb332e583c7a0586a9
    bnx2: Fix netpoll crash.

If ->open() and ->close() are called multiple times, the same napi structs
will be added to dev->napi_list multiple times, corrupting the dev->napi_list.
This causes free_netdev() to hang during rmmod.

We fix this by calling netif_napi_del() during ->close().

Also, bnx2_init_napi() must not be in the __devinit section since it is
called by ->open().

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>