pinctrl: mediatek: add a check for error in mtk_pinconf_bias_get_rsel()

pinctrl: mediatek: add a check for error in mtk_pinconf_bias_get_rsel()

All the other mtk_hw_get_value() calls have a check for "if (err)" so
we can add one here as well.  This silences a Smatch warning:

    drivers/pinctrl/mediatek/pinctrl-mtk-common-v2.c:819 mtk_pinconf_bias_get_rsel()
    error: uninitialized symbol 'pd'.

Fixes: fb34a9ae383a ("pinctrl: mediatek: support rsel feature")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20211127140836.GB24002@kili
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>