wifi: mt76: mt7921: fix missing unwind goto in `mt7921u_probe`

`mt7921u_dma_init` can only return zero or negative number according to its
definition. When it returns non-zero number, there exists an error and this
function should handle this error rather than return directly.

Fixes: 0d2afe09 ("mt76: mt7921: add mt7921u driver")
Signed-off-by: Jiefeng Li <jiefeng_li@hust.edu.cn>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>