media: v4l2-compat-ioctl32: simplify casts

Making the cast right for get_user/put_user is not trivial, as
it needs to ensure that the types are the correct ones.

Improve it by using macros.

Tested with vivid with:
	$ sudo modprobe vivid no_error_inj=1
	$ v4l2-compliance-32bits -a -s10 >32bits && v4l2-compliance-64bits -a -s10 > 64bits && diff -U0 32bits 64bits
	--- 32bits	2018-04-17 11:18:29.141240772 -0300
	+++ 64bits	2018-04-17 11:18:40.635282341 -0300
	@@ -1 +1 @@
	-v4l2-compliance SHA   : bc71e4a67c6fbc5940062843bc41e7c8679634ce, 32 bits
	+v4l2-compliance SHA   : bc71e4a67c6fbc5940062843bc41e7c8679634ce, 64 bits

Using the latest version of v4l-utils with this patch applied:
	https://patchwork.linuxtv.org/patch/48746/



Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>