gpio: omap: Fix GPIO numbering for deferred probe

If gpio-omap probe fails with -EPROBE_DEFER, the GPIO numbering
keeps increasing. Only increase the gpio count if gpiochip_add()
was successful as otherwise the numbers will increase for each
probe attempt.

Cc: Javier Martinez Canillas <javier@dowhile0.org>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Santosh Shilimkar <ssantosh@kernel.org>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>