iio: magnetometer: mmc35240: Use a smaller sleep value

According to datasheet, Page 8, minimum wait time to complete
measurement is 10ms. Adjusting this value will increase the
userspace polling rate.

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>