wifi: mt76: mt7915: add bf backoff limit table support

wifi: mt76: mt7915: add bf backoff limit table support

The commit 22b980badc0f ("mt76: add functions for parsing rate power limits
from DT") introduced generic support for rates limits in the devicetree.
But the mt7915 supports beamforming and has another table for configuring
the backoff limits. These can be configured in the DT with the paths-*
properties. The path-*-bf are the ones relevant for beamforming and the
ones without -bf suffix for "traditional" path backoff.

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Sven Eckelmann (Plasma Cloud) <se@simonwunderlich.de>
Link: https://patch.msgid.link/20251007-backoff-table-support-v3-3-fd6e2684988f@simonwunderlich.de
Signed-off-by: Felix Fietkau <nbd@nbd.name>