drm/bridge: tc358768: Fix tc358768_ns_to_cnt()

The tc358768_ns_to_cnt() is, most likely, supposed to do a div-round-up
operation, but it misses subtracting one from the dividend.

Fix this by just using DIV_ROUND_UP().

Fixes: ff1ca639 ("drm/bridge: Add tc358768 driver")
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Tested-by: Maxim Schwalm <maxim.schwalm@gmail.com> # Asus TF700T
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230906-tc358768-v4-11-31725f008a50@ideasonboard.com