mmc: mtk-sd: increase the minimum bus frequency

With a 48MHz input clock, the lowest bus frequency can be as low as
48000000 / (4 * 4095) = 2930Hz. Such an extremely low frequency will cause
the mmc framework take seconds to finish the initialization.

Limiting the minimum bus frequency to a slightly higher value can solve the
issue without any side effects.

Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>