iio: magnetometer: mmc35240: i2c device name should be lower case

This is the standard convention for i2c device name and
also this is the name used in some Intel platforms DT
files.

Fixes: abeb6b1e ("iio: magnetometer: Add support for MEMSIC MMC35240")
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>