wifi: mt76: mt7996: fix uninitialized symbol warning

wifi: mt76: mt7996: fix uninitialized symbol warning

Fix below smatch warnings:
drivers/net/wireless/mediatek/mt76/mt7996/main.c:952 mt7996_mac_sta_add_links()
error: uninitialized symbol 'err'.
drivers/net/wireless/mediatek/mt76/mt7996/main.c:1133 mt7996_set_rts_threshold()
error: uninitialized symbol 'ret'.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/r/202504101051.1ya4Z4va-lkp@intel.com/
Signed-off-by: sunliming <sunliming@kylinos.cn>
Link: https://patch.msgid.link/20250419031528.2073892-1-sunliming@linux.dev
Signed-off-by: Felix Fietkau <nbd@nbd.name>