wimax/i2400m: don't reset device on i2400m_dev_shutdown()

i2400m_dev_shutdown() tried to reset the device to put it in a known
state before shutting down.

But that turned out to be pointless. We reach this case in two paths:

 1 - when the device resets, to clean up state
 2 - when the driver is unloaded, for the same

however, in both cases it is pointless; in (1) the device is already
reset, why do it again? in (2) we can't -- the USB stack, for example,
doesn't allow communicating with the device when the driver is being
unbound and if the device is disconnected, the device is gone already.

So just remove it. Leave the function as a placeholder for future
cleanups that will be done from data allocated by the driver during
device operation.

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