rk_pinctrl: fix error check

The parse_bias method returns a signed int, with a value of -1 when
the device tree reports nothing of the bias configuration. Convert the
local 'bias' from unsigned to signed to properly check this condition.

PR: 229721
Reviewed by: mhorne
MFC after: 3 days

(cherry picked from commit 97ab935d566c85458bc7cab99c402f026d704eff)