mt76: mt7915: fix rx fcs error count in testmode

FCS error packets are filtered by default and won't be reported to
driver, so that RX fcs error and PER in testmode always show zero.
Fix this issue by reading fcs error count from hw counter.

We did't fix this issue by disabling fcs error rx filter since it may
let HW suffer some SER errors.

Fixes: 5d8a83f0 ("mt76: mt7915: implement testmode rx support")
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>