maestro3: treat firmware data as const

The maestro3 driver is byte-swapping its firmware to be host-endian in
advance, when it doesn't seem to be necessary -- we could just use
le16_to_cpu() as we load it.

Doing that means that we need to switch the in-tree firmware to be
little-endian too.

Take the least intrusive way of doing this, which is to switch the
existing snd_m3_convert_from_le() function to convert _to_ little-endian
instead, and use it on the in-tree firmware instead of the loaded
firmware. It's a bit suboptimal but doesn't matter much right now
because we're about to remove the special cases for the in-tree version
anyway.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>