wifi: mt76: mt7996: fix potential tx_retries underflow

wifi: mt76: mt7996: fix potential tx_retries underflow

When FIELD_GET returns 0 for the retry count, subtracting 1 causes
an unsigned integer underflow, resulting in tx_retries becoming a
very large value (0xFFFFFFFF for u32).

Fix by checking if count is non-zero before subtracting 1.

Fixes: 2461599f835e ("wifi: mt76: mt7996: get tx_retries and tx_failed from txfree")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://patch.msgid.link/20260605113306.3485554-4-ryder.lee@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>