drivers/sensor: lsm6dso: Remove unneeded read/modify/write in shub read

The code in the ST HAL does a read/modify/write to change the bits in
the LSM6DSO_FUNC_CFG_ACCESS register that control which register bank is
active.

All the other bits in the register are defined as zero.  It's possible
to simply set the register to the desired value without reading the
contents first.

This bank switch needs to be done twice for every sensor read when the
sensor hub is used.  The driver as it is can not keep up with the higher
update rates of the lsm6dso.  So any speed increase in this code allows
for a higher update rate as well as reduced latency.

Previously, a read of the lsm6dso's accel and gyro on a 400 kHz I2C bus
with a 3-axis magnetometer on the sensor hub takes 2.69 ms.  This drops
that to 2.26 ms.  This is enough to support the 417 Hz ODR.

Signed-off-by: Trent Piepho <trent.piepho@igorinstitute.com>