pinctrl: fix pinconf_dbg_config_write return type

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]

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>