iio:adc:ad7923: Put macro argument between ()'s

Put macro argument between parenthesis in order to avoid precedence
issues. Solves the following checkpath.pl's messages:

CHECK: Macro argument 'mode' may be better as '(mode)' to avoid
precedence issues
CHECK: Macro argument 'channel' may be better as '(channel)' to
avoid precedence issues
CHECK: Macro argument reuse 'sequence' - possible side-effects?
CHECK: Macro argument 'sequence' may be better as '(sequence)' to
avoid precedence issues
CHECK: Macro argument 'val' may be better as '(val)' to avoid
precedence issues
CHECK: Macro argument 'dec' may be better as '(dec)' to avoid precedence
issues
CHECK: Macro argument 'bits' may be better as '(bits)' to avoid
precedence issues

Signed-off-by: Bárbara Fernandes <barbara.fernandes@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>