drivers: sensor: lis3mdl: always use continuous mode

The LIS3MDL driver misunderstands the MD[1:0] bits in CTRL_REG3.
Correct this so that all ODR settings are functional.

Currently, single-conversion mode is enabled when fast ODR is set,
resulting in only one measurement being taken before the device
immediately returns to power-down mode. The result is that all fast-ODR
rates (155 and higher) are not usable when assigned to CONFIG_LIS3MDL_ODR.

Continuous mode is the only mode truly supported by this driver and should
always be used.

Signed-off-by: Aedan Cullen <aedan@aedancullen.com>