wifi: mt76: mt7921: reduce the size of MCU firmware download Rx queue

We actually don't need the reserve the 512 entries for the MCU firmware
download Rx queue because the queue was only used in the firmware download
phase to save the most of space and the reduction can significantly help
with reducing latency we spent by ~20% further in resetting the Rx queue
as the device was waking up from deep sleep mode.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>