mt76: mt7615: Fix a dereference of pointer sta before it is null checked

Currently the assignment of idx dereferences pointer sta before
sta is null checked, leading to a potential null pointer dereference.
Fix this by assigning idx when it is required after the null check on
pointer sta.

Addresses-Coverity: ("Dereference before null check")
Fixes: a4a5a430

 ("mt76: mt7615: fix TSF configuration")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>