The lowest supported clock frequency of the PHY is 125MHz (see also
mtk_mipi_tx_pll_enable()), but the clamping in .round_rate() has the
wrong minimal value, which will make the .enable() op return -EINVAL on
low frequencies. Fix the minimal clamping value.
Fixes: efda51a58b4a ("drm/mediatek: add mipi_tx driver for mt8183")
Signed-off-by: Michael Walle <mwalle@xxxxxxxxxx>