clk: microchip: core: correct return value on *_get_parent()

clk: microchip: core: correct return value on *_get_parent()

roclk_get_parent() and sclk_get_parent() has the possibility of
returning -EINVAL, however the framework expects this call to always
succeed since the return value is unsigned.

If there is no parent map defined, then the current value programmed in
the hardware is used. Let's use that same value in the case where
-EINVAL is currently returned.

This index is only used by clk_core_get_parent_by_index(), and it
validates that it doesn't overflow the number of available parents.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202512050233.R9hAWsJN-lkp@intel.com/
Signed-off-by: Brian Masney <bmasney@redhat.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Link: https://lore.kernel.org/r/20251205-clk-microchip-fixes-v3-2-a02190705e47@redhat.com
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>