clk: sprd: return correct type of value for _sprd_pll_recalc_rate

clk: sprd: return correct type of value for _sprd_pll_recalc_rate

The function _sprd_pll_recalc_rate() defines return value to unsigned
long, but it would return a negative value when malloc fail, changing
to return its parent_rate makes more sense, since if the callback
.recalc_rate() is not set, the framework returns the parent_rate as
well.

Fixes: edb8737dcd4b ("clk: sprd: add adjustable pll support")
Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Link: https://lkml.kernel.org/r/20200519030036.1785-2-zhang.lyra@gmail.com
Reviewed-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>