mt76: mt7915: fix endianness warnings in mt7915_debugfs_rx_fw_monitor

Fix the following sparse warnings:
warning: incorrect type in initializer (different base types)
   expected restricted __le16 [usertype] msg_type
   got int
warning: incorrect type in assignment (different base types)
   expected restricted __le32 [usertype] timestamp
   got unsigned int

Fixes: 988845c9

 ("mt76: mt7915: add support for passing chip/firmware debug data to user space")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>