iio: accel: mma9553: check for error in reading initial activity and stepcnt

When configuring gpio, we need to read initial values for activity and
step count. This function may fail due to i2c read errors.

Check the error code returned by mma9553_read_activity_stepcnt
and return the appropriate error in gpio config function.

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