clk: renesas: rzg2l: Fix return value and unused assignment

Currently the function returns NULL on error, so exact error code is
lost.  This patch changes return convention of the function to use
ERR_PTR() on error instead.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Link: https://lore.kernel.org/r/1623896524-102058-1-git-send-email-yang.lee@linux.alibaba.com


[geert: Drop curly braces]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>