staging: iio: adc: add spaces around binary operators

Add spaces around binary operators like -, +, * etc
in order to improve readability

Signed-off-by: Ioana Ciornei <ciorneiioana@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>