Simplify the size argument of kzalloc() memory allocation by using
sizeof(*ptr) syntax in adc.c and devs.c.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
[krzk: Rewrite commit message]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>