media: em28xx.h: Fix most coding style issues

There used to have a lot of coding style issues there. The
ones detected by checkpatch, in strict mode, got fixed.

Still, we need to work more on it, in order to document all
struct fields using kernel-doc macros, but this will be done
on some future patch.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>