drivers: sensor: bme280: fix "config" register intialization issue

A write to the "ctrl_meas" register can cause the sensor to transition
from sleep to normal mode (which is default Kconfig settings). As per
BME280 datasheet, writes to the "config" register in normal mode may be
ignored. This can lead to use BME280_STANDBY_05MS and BME280_FILTER_OFF
instead of the values set by the application. To fix this, write to the
"config" register before "ctrl_meas" in the sensor init function.

Signed-off-by: Loic Domaigne <tech@domaigne.com>