staging:iio:adc:spear rename device specific state structure to _state

Also rename instances of it to st.  This brings the driver inline with
the conventions of IIO and avoids some naming confusion with some IIO
structures which also use the _info postfix.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>