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]

Change-Id: Iff411ac803e46070fafce5c2d23626c890856bf2
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>
Git-commit: 3b59e432ec9c093311e0ad057e330e04feb79007
Git-Repo: git://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/


Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>