mt76: drop MCU header size from buffer size in __mt76_mcu_send_firmware

Each firmware piece with mt7915 and mt7921 do not have the MCU header to
reroute the packet to the internal RAM. So we do not need to consider the
MCU header size in __mt76_mcu_send_firmware.
Move the MCU header overhead calculation to mt76_mcu_send_firmware (for
older chipsets)

Fixes: e57b7901 ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Fixes: 1c099ab4 ("mt76: mt7921: add MCU support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>