pinctrl: spacemit: fix NULL check in spacemit_pin_set_config

In the Linux kernel, the following vulnerability has been resolved:

pinctrl: spacemit: fix NULL check in spacemit_pin_set_config

spacemit_pin_set_config() looks up the per-pin descriptor with
spacemit_get_pin() then checks the wrong variable for failure:

const struct spacemit_pin *spin = spacemit_get_pin(pctrl, pin);
...
if (!pin)
return -EINVAL;

reg = spacemit_pin_to_reg(pctrl, spin->pin);

pin is an unsigned int pin id, where 0 (GPIO_0 / gmac0_rxdv on K3) is a
valid pin, so rejecting it here drops the PAD config write for the first
pin of every group. On K3 Pico-ITX the GMAC RGMII group lists pin 0 as
its first entry, so its drive-strength / bias configuration was silently
ignored.

The intended guard is against spacemit_get_pin() returning NULL when the
pin id isn't in the SoC's pin table. Check spin instead, which both
restores PAD setup for pin 0 and prevents a NULL deref on spin->pin.