Have pinconf_dbg_config_write() return a ssize_t. This fixes the following
compilation warning:
drivers/pinctrl/pinconf.c:617:2: warning: initialization from incompatible pointer type [enabled by default]
drivers/pinctrl/pinconf.c:617:2: warning: (near initialization for ‘pinconf_dbg_pinconfig_fops.write’) [enabled by default]