wimax/i2400m: cleanup initialization/destruction flow

Currently the i2400m driver was starting in a weird way: registering a
network device, setting the device up and then registering a WiMAX
device.

This is an historic artifact, and was causing issues, a some early
reports the device sends were getting lost by issue of the wimax_dev
not being registered.

Fix said situation by doing the wimax device registration in
i2400m_setup() after network device registration and before starting
thed device.

As well, removed spurious setting of the state to UNINITIALIZED;
i2400m.dev_start() does that already.

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