mt76: fix returnvar.cocci warnings

drivers/net/wireless/mediatek/mt76/mt76x2_main.c:86:5-8: Unneeded variable: "ret". Return "0" on line 112

 Remove unneeded variable used to store return value.

Generated by: scripts/coccinelle/misc/returnvar.cocci

Fixes: a5f6039c8f9c ("mt76: add driver code for MT76x2e")
CC: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>