Staging: iio: Prefer using the BIT macro

This patch replaces bit shifting on 1 with the BIT(x) macro
as it's extensively used by other function in this driver.

This was done with coccinelle:
@@ int g; @@

-(1 << g)
+BIT(g)

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>