Bluetooth: mgmt: remove NULL check in add_ext_adv_params_complete()

Remove the cmd pointer NULL check in add_ext_adv_params_complete()
because it occurs earlier in add_ext_adv_params(). This check is
also unnecessary because the pointer is dereferenced just before it.

Found by Linux Verification Center (linuxtesting.org) with Svace.

Signed-off-by: Roman Smirnov <r.smirnov@omp.ru>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>