drm/bridge: tc358768: fix PLL parameters computation

According to Toshiba documentation the PLL input clock after the divider
should be not less than 4MHz, fix the PLL parameters computation
accordingly.

Fixes: ff1ca639 ("drm/bridge: Add tc358768 driver")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-3-francesco@dolcini.it