sdp: check if sdp device is actually present in sdp_remove_one

sdp: check if sdp device is actually present in sdp_remove_one

If sdp fails to initialize at driver startup for any reason,
the device is still registered with the ib_core, but there will be
no client data (i.e., ib_set_client_data() will not be called, and all
kernel resources are de-allocated).

On removal, ib_get_client_data() will return NULL in this case -- and this
must be tested for -- or we will get a kernel Oops for a NULL pointer
dereference.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>