drm/bridge: sil_sii8620: initialize return of sii8620_readb

clang static analysis flags this error

sil-sii8620.c:184:2: warning: Undefined or garbage value
  returned to caller [core.uninitialized.UndefReturn]
        return ret;
        ^~~~~~~~~~

sii8620_readb calls sii8620_read_buf.
sii8620_read_buf can return without setting its output
pararmeter 'ret'.

So initialize ret.

Fixes: ce6e153f ("drm/bridge: add Silicon Image SiI8620 driver")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200712152453.27510-1-trix@redhat.com