iio: adc: ingenic-adc: use guard()() and scoped_guard() to handle synchronisation

Replace mutex_lock() and mutex_unlock() calls with guard()() in
functions ingenic_adc_set_adcmd(), ingenic_adc_set_config(),
ingenic_adc_enable(), ingenic_adc_capture(), and with scoped_guard()
in function ingenic_adc_read_chan_info_raw().

This removes the need to call the unlock function, as the lock is
automatically released when the function return or the scope exits
for any other case.

Signed-off-by: Felipe Ribeiro de Souza <felipers@ime.usp.br>
Co-developed-by: Lucas Ivars Cadima Ciziks <lucas@ciziks.com>
Signed-off-by: Lucas Ivars Cadima Ciziks <lucas@ciziks.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>