drm/bridge: it6505: Fix the read buffer array bound

The size of read_buf is READ_BUFFER_SIZE (200), so we can't access it
with read_buf + PAGE_SIZE (4096). Extend the READ_BUFFER_SIZE to 400 and
set the end position to read_buf + READ_BUFFER_SIZE.

Fixes: b5c84a9e ("drm/bridge: add it6505 driver")
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220228081421.1504213-1-hsinyi@chromium.org