pinctrl: pinmux: handle radix_tree_insert() errors in pinmux_generic_add_function()

pinctrl: pinmux: handle radix_tree_insert() errors in pinmux_generic_add_function()

pinctrl_generic_add_function() doesn't check result of radix_tree_insert()
despite they both may return a negative error code.  Linus Walleij said he
has copied the radix tree code from kernel/irq/ where the functions calling
radix_tree_insert() are *void* themselves; I think it makes more sense to
propagate the errors from radix_tree_insert() upstream if we can do that...

Found by Linux Verification Center (linuxtesting.org) with the Svace static
analysis tool.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Link: https://lore.kernel.org/r/20230719202253.13469-4-s.shtylyov@omp.ru
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>