pinctrl: imx: Return NULL if no group is matched and found

Currently if no group is matched and found, this function will return
the last grp to the caller, this is not expected, it is supposed to
return NULL in this case.

Fixes: e566fc11 ("pinctrl: imx: use generic pinctrl helpers for managing groups")
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/20250327031600.99723-1-hui.wang@canonical.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>