staging:iio:ade7753/ade7754/ade7759: Use spi_w8r16be() instead of spi_w8r16()

Using spi_w8r16be() will do the conversion of the result from big endian to
native endian in the helper function. This makes the code a bit smaller and also
keeps sparse happy. Fixes the following sparse warnings:

	drivers/staging/iio/meter/ade7753.c:97:29: warning: incorrect type in argument 1 (different base types)
	drivers/staging/iio/meter/ade7753.c:97:29:    expected restricted __be16 const [usertype] *p
	drivers/staging/iio/meter/ade7753.c:97:29:    got unsigned short [usertype] *val

	drivers/staging/iio/meter/ade7754.c:97:29: warning: incorrect type in argument 1 (different base types)
	drivers/staging/iio/meter/ade7754.c:97:29:    expected restricted __be16 const [usertype] *p
	drivers/staging/iio/meter/ade7754.c:97:29:    got unsigned short [usertype] *val

	drivers/staging/iio/meter/ade7759.c:97:29: warning: incorrect type in argument 1 (different base types)
	drivers/staging/iio/meter/ade7759.c:97:29:    expected restricted __be16 const [usertype] *p
	drivers/staging/iio/meter/ade7759.c:97:29:    got unsigned short [usertype] *val

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Mark Brown <broonie@linaro.org>