staging: iio: add proper SPDX identifiers to remaining driver files

There are a number of IIO staging drivers that do not have a proper SPDX
identifier on it.  So fix that up and at the same time, remove the "free
form" license text, as that's pretty much impossible for any tool to
parse.

Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>