wifi: mt76: mt7921: fix use after free in mt7921_acpi_read()

Don't dereference "sar_root" after it has been freed.

Fixes: f965333e ("mt76: mt7921: introduce ACPI SAR support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>