iio:adc:ad7923: Use BIT macro instead of bitshift

Replace use of the operation '<<' by the BIT macro. Solves checkpath.pl's
message:

CHECK: Prefer using the BIT macro

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