pinmux_generic_get_function() can return NULL and the pointer 'function'
was dereferenced without checking against NULL. Add checking of pointer
'function' in pcs_get_function().

Found by code review.

Cc: stable@vger.kernel.org
Fixes: 571aec4d ("pinctrl: single: Use generic pinmux helpers for managing functions")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Link: https://lore.kernel.org/20240808041355.2766009-1-make24@iscas.ac.cnSigned-off-by: Linus Walleij <linus.walleij@linaro.org>