staging: iio: adc: ad7816.c: Replace bit shifting with BIT macro

Challenge suggested by coccinelle.
Prefer using BIT and replace bit shifting with the BIT(x) macro.

Signed-off-by: Payal Kshirsagar <payal.s.kshirsagar.98@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>