video: ipu_common: fix build error

Some toolchains fail to build
"clk->rate = (u64)(clk->parent->rate * 16) / div;"
And the cast usage is wrong.

Use the following code to fix the issue,
"
  do_div(parent_rate, div);
  clk->rate = parent_rate;
"

Reported-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>