pwm: spear: fix check on pwmchip_add() return value

pwmchip_add() returns zero on success and a negative value on error,
so the condition of the check must be inverted.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>