Publish available gain values in sysfs

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Fix misplaced break (in ads124x_write_raw's switch)

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Read gain values from sysfs in "human format"

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>