drm/sun4i: dsi: Fix unitialized variable warning

Since the DPHY rework, one error path doesn't set the return error code
before jumping to its error label, which in turns make gcc (rightfully)
complain about the variable holding the error code being uninitialized.

Fix this.

Fixes: bb3b6fcb ("sun6i: dsi: Convert to generic phy handling")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208090540.19626-1-maxime.ripard@bootlin.com