mmc: litex_mmc: Use DIV_ROUND_UP for more accurate clock calculation

mmc: litex_mmc: Use DIV_ROUND_UP for more accurate clock calculation

The previous clock uses roundup_pow_of_two() to calculate the core
clock frequency. It does not meet the actual hardware meaning.
The actual frequency is calculated by "ref_clk / ((div >> 1) << 1)".

Fix the clock divider calculation.

Fixes: 92e099104729 ("mmc: Add driver for LiteX's LiteSDCard interface")
Signed-off-by: Inochi Amaoto <inochiama@gmail.com>
Reviewed-by: Gabriel Somlo <gsomlo@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulfh@kernel.org>