bme680: Rework setting the spi_mem_page accordingly to the datasheet

According to the BME680 datasheet, only the spi_mem_page bit can be
changed in its 8-bit register. Therefore we shouldn't just write 0s
to the remaining bits, but update the register content in read-modify-write
fashion instead.
Rework the bme680_set_mem_page(), bme680_init() and register-access
macros.

Signed-off-by: Jan Kuliga <jtkuliga@gmail.com>