input: mxc6225: Fixed writing of detection register

The register number was wrong. Also there is no need to preserve
pre-existing values in this register.
input: mxc6225: Read x and y coordinates in a single transfer

This cuts down considerably on the amount activity on the I2C bus.
input: mxc6225: Handle I2C transfer errors
input: mxc6225: Check I2C adapter functionality in probe