pinctrl: fix buffer size for pinctrl_generic_set_state_prefix()

This buffer has the concatenated prefix and name written into it, so it
must be large enough to cover both strings plus the terminating NUL.

Fixes: 92c4a95e ("pinctrl: Add new function pinctrl_generic_set_state_prefix()")
Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>