mt76: fix a leaked reference by adding a missing of_node_put

mt76: fix a leaked reference by adding a missing of_node_put

The call to of_find_node_by_phandle returns a node pointer with refcount
incremented thus it must be explicitly decremented after the last
usage.

Detected by coccinelle with the following warnings:
./drivers/net/wireless/mediatek/mt76/eeprom.c:58:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 48, but without a corresponding object release within this function.
./drivers/net/wireless/mediatek/mt76/eeprom.c:61:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 48, but without a corresponding object release within this function.
./drivers/net/wireless/mediatek/mt76/eeprom.c:67:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 48, but without a corresponding object release within this function.
./drivers/net/wireless/mediatek/mt76/eeprom.c:70:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 48, but without a corresponding object release within this function.
./drivers/net/wireless/mediatek/mt76/eeprom.c:72:1-7: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 48, but without a corresponding object release within this function.

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: Felix Fietkau <nbd@nbd.name>
Cc: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>