i2c-gpio: Fix adapter number

It turns out that platform_device.id is a "u32" so testing it for being
nonnegative is useless when setting up an i2c bitbang device.  Instead,
do what the platform_bus code does and test it against the value "-1". 

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Jean Delvare <khali@linux-fr.org>