video: adf: remove PAGE_SIZE from userspace-facing header

Systems may define PAGE_SIZE in userspace limits.h but don't have to.
PAGE_SIZE was picked as an arbitrary "reasonable" limit so just use 4096
instead.

Change-Id: I9555e39aba64a3a70f61eb6ded2a4129ab236ce0
Signed-off-by: Greg Hackmann <ghackmann@google.com>