clk: mediatek: clk-mt8173-apmixedsys: Fix return value for of_iomap() error

clk: mediatek: clk-mt8173-apmixedsys: Fix return value for of_iomap() error

The of_iomap() function returns NULL in case of error so usage of
PTR_ERR() is wrong!
Change that to return -ENOMEM in case of failure.

Fixes: 41138fbf876c ("clk: mediatek: mt8173: Migrate to platform driver and common probe")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230615122051.546985-3-angelogioacchino.delregno@collabora.com
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>