drm/bridge: ps8640: Fix size mismatch warning w/ len

After commit 26195af5 ("drm/bridge: ps8640: Drop the ability of
ps8640 to fetch the EDID"), I got an error compiling:

  error: comparison of distinct pointer types
  ('typeof (len) *' (aka 'unsigned int *') and
   'typeof (msg->size) *' (aka 'unsigned long *'))
  [-Werror,-Wcompare-distinct-pointer-types]

Fix it by declaring the `len` as size_t.

The above error only shows up on downstream kernels without commit
d03eba99 ("minmax: allow min()/max()/clamp() if the arguments have
the same signedness."), but since commit 26195af5 ("drm/bridge:
ps8640: Drop the ability of ps8640 to fetch the EDID") is a "Fix" that
will likely be backported it seems nice to make it easy. ...plus it's
more correct to declare `len` as size_t anyway.

Fixes: 26195af5

 ("drm/bridge: ps8640: Drop the ability of ps8640 to fetch the EDID")
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231218090454.1.I5c6eb80b2f746439c4b58efab788e00701d08759@changeid