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: NLorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: NFelix Fietkau <nbd@nbd.name>