iio: accel: mma9553: fix gpio bitnum init value

Initial value of gpio bitnum is set to -1, but
the variable is declared as unsigned.

Use a positive invalid value for initial gpio
bitnum.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Suggested-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>