baylibre-acme: Optimize reading of values from sysfs.

Opening a file has a cost (security, allocation, syscalls). The
read_sample() function always does an open/read/close sequence.

In order to optimize that, let's open the file at the moment the
acquisition starts, close it when the acquisition stops and make
read_sample() only lseek() to the beginning of the file and read
the value.

Signed-off-by: Daniel Lezcano <redacted>
Signed-off-by: Bartosz Golaszewski <redacted>