staging: iio: Replace a bit shift by a use of BIT.

This patch replaces bit shifting on 1 with the BIT(x) macro.
This was done with coccinelle:
@@
constant c;
@@

-1 << c
+BIT(c)

Signed-off-by: Arushi Singhal <arushisinghal19971997@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>