mt76: mt7915: fix endianness in mt7915_rf_regval_get

Fix the following sparse warning in mt7915_rf_regval_get routine:
drivers/net/wireless/mediatek/mt76/mt7915/debugfs.c:979:16: warning: cast to restricted __le32

Fixes: 0a17329a

 ("mt76: mt7915: add debugfs knob for RF registers read/write")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>