Cleanup

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Refactor ads124x_set_pga_gain (remove ads124x_get_pga_gain)

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Move locking to ads124x_select_input

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Format code according to Linux coding standards

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