staging: iio: if(__LITTLE_ENDIAN) -> #ifdef __LITTLE_ENDIAN

Unsuprisingly this symbol isn't defined on big endian systems.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>