backlight: atmel-pwm-bl: clean up probe error handling

Clean up probe error handling by checking parameters before any
allocations and removing an obsolete error label.  Also remove unnecessary
reset of private gpio number.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>