iio:adc:ad7887 Fix channel reported endianness from cpu to big endian

Note this also sets the endianness to big endian whereas it would
previously have defaulted to the cpu endian.  Hence technically
this is a bug fix on LE platforms.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: stable@vger.kernel.org