wifi: mt76: mt7996: Add NULL check in mt7996_thermal_init

wifi: mt76: mt7996: Add NULL check in mt7996_thermal_init

devm_kasprintf() can return a NULL pointer on failure,but this
returned value in mt7996_thermal_init() is not checked.
Add NULL check in mt7996_thermal_init(), to handle kernel NULL
pointer dereference error.

Fixes: 69d54ce7491d ("wifi: mt76: mt7996: switch to single multi-radio wiphy")
Signed-off-by: Charles Han <hanchunchao@inspur.com>
Link: https://patch.msgid.link/20250407095551.32127-1-hanchunchao@inspur.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>