samples: accel_polling: set sampling frequency when necessary

The accel_polling sample uses various sensor, but doesn't set a sampling
rate. But some sensors (like st,lsm6dso) have a default sampling
frequency of 0. So, depending on the sensor, the sample may not always
work.

There are two ways to fix this: either all drivers must set a valid
sampling rate, or the sample shall at least try to set a value if there
is none.

We propose here the second approach, wich should allow the sample to
work on more sensors out of the box.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>