wimax/i2400m: make i2400m->bus_dev_{stop,start}() optional

In coming commits, the i2400m SDIO driver will not use
i2400m->bus_dev_stop().

Thus changed to check before calling, as an empty stub has more
overhead than a call to check if the function pointer is non-NULL.

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