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

commit e39d99059ad7f75d7ae2d3c59055d3c476cdb0d9 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>