mt76: mt7615: fix sparse warnings: incorrect type in assignment (different base types)

Fix the following sparse warning in mt7615_mcu_bss_info_ext_header:
drivers/net/wireless/mediatek/mt76/mt7615/mcu.c:728:30: sparse: sparse:
incorrect type in assignment (different base types)
drivers/net/wireless/mediatek/mt76/mt7615/mcu.c:728:30: sparse:
expected restricted __le32 [usertype] mbss_tsf_offset

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 04b8e659 ("mt76: add mac80211 driver for MT7615 PCIe-based chipsets")
Fixes: 7339fbc0caa5 ("mt7615: mcu: do not use function pointers whenever possible")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>