drm/bridge: sii902x: fix comparision of u32 with less than zero

The less than check for the variable num_lanes is always going to be
false because the variable is a u32.  Fix this by making num_lanes an
int and also make loop index i an int too.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: ff578163 ("drm/bridge: sii902x: Implement HDMI audio support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190603142102.27191-1-colin.king@canonical.com