pinctrl/pinctrl-spear: remove IS_ERR checking of pmx->pctl

pinctrl_register returns a pointer of struct type struct pinctrl_dev,
if successfully registered to pinctrl subsystem, otherwise returns
NULL, and there wont' be any pointers which are not dereferencible.

They are not type of pointer addresses but are kind of error
codes rather actual addresses, but are a kind of return
codes of functions returning integer types.

return -ENODEV if device registration fails.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>