pinctrl: samsung: don't truncate the last char

We were allocating enough space because sizeof("-grp") and
sizeof("-mux") are both equal to 5 but in the snprintf() we only allowed
for 4 characters so the last 'p' and 'x' characters were truncated.

The allocate and sprintf can be done in one step with the kasprintf().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>