gpio: adp5588-gpio: gpio_start must be signed

Common code interprets this as a signed value (a negative value is used to
request dynamic ID allocation), so make sure the platform data has proper
types to support that.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>