iio: pressure: bmp280 endian tidy ups

iio: pressure: bmp280 endian tidy ups

There is a somewhat interesting mixture of be16 and le16 going on in
this one function.  Changes here formalize that a little more.

CHECK   drivers/iio/pressure/bmp280-core.c
drivers/iio/pressure/bmp280-core.c:215:35: warning: cast to restricted __le16
drivers/iio/pressure/bmp280-core.c:229:37: warning: cast to restricted __be16
drivers/iio/pressure/bmp280-core.c:229:37: warning: cast to restricted __be16
drivers/iio/pressure/bmp280-core.c:229:37: warning: cast to restricted __be16
drivers/iio/pressure/bmp280-core.c:229:37: warning: cast to restricted __be16
drivers/iio/pressure/bmp280-core.c:230:36: warning: cast to restricted __be16
drivers/iio/pressure/bmp280-core.c:230:36: warning: cast to restricted __be16
drivers/iio/pressure/bmp280-core.c:230:36: warning: cast to restricted __be16
drivers/iio/pressure/bmp280-core.c:230:36: warning: cast to restricted __be16
drivers/iio/pressure/bmp280-core.c:237:37: warning: cast to restricted __le16

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>