clk: fractional-divider: Split out clk_fd_get_div() helper

Split out clk_fd_get_div() helper for the future use elsewhere.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20221122130732.48537-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>