pinctrl: ingenic: Fix group & function error checking

Commit a203728a ("pinctrl: core: Return selector to the pinctrl
driver") and commit f913cfce ("pinctrl: pinmux: Return selector to
the pinctrl driver") modified the return values of
pinctrl_generic_add_group() and pinmux_generic_add_function()
respectively, but did so without updating their callers. This broke the
pinctrl-ingenic driver, which treats non-zero return values from these
functions as errors & fails to probe. For example on a MIPS Ci20:

  pinctrl-ingenic 10010000.pin-controller: Failed to register group uart0-hwflow
  pinctrl-ingenic: probe of 10010000.pin-controller failed with error 1

Without the pinctrl driver probed, other drivers go on to fail to probe
too & the system is unusable.

Fix this by modifying the error checks to treat only negative values as
errors, matching the commits that introduced the breakage & similar
changes made to other drivers.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: a203728a ("pinctrl: core: Return selector to the pinctrl driver")
Fixes: f913cfce ("pinctrl: pinmux: Return selector to the pinctrl driver")
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Tony Lindgren <tony@atomide.com>
Cc: linux-gpio@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>