iio: adc: ti-ads1298: Add parentheses around macro parameter

iio: adc: ti-ads1298: Add parentheses around macro parameter

ADS1298_REG_CHnSET() is missing parentheses around the parameter 'n'.
Add them to follow kernel macro coding style and prevent potential
operator precedence issues if the argument is an expression.

Signed-off-by: Md Shofiqul Islam <shofiqtest@gmail.com>
Reviewed-by: Stepan Ionichev <sozdayvek@gmail.com>
Acked-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>