wimax/i2400m: implement passive mode as a module option

Some versions of the user space Intel WiMAX daemon need to have full
control over the device initialization sequence. By setting the module
option i2400.passive_mode to 1, the driver defers all device
configuration and initialization to user space.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>