mt76: fix a possible NULL pointer dereference in mt76_testmode_dump

Fix a possible NULL pointer dereference in mt76_testmode_dump() since
nla_nest_start returns NULL in case of error

Fixes: f0efa862

 ("mt76: add API for testmode support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>