clk: ti: omap3+: dpll: use non-locking version of clk_get_rate

As the code in this file is being executed within irq context in some
cases, we must avoid the clk_get_rate which uses mutex internally.
Switch the code to use clk_hw_get_rate instead which is non-locking.

This fixes an issue where PM runtime will hang the system if enabled
with a serial console before a suspend-resume cycle.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Fixes: a53ad8ef ("clk: ti: Convert to clk_hw based provider APIs")
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>