wimax: oops: wimax_dev_add() is the only one that can initialize the state

When a new wimax_dev is created, it's state has to be __WIMAX_ST_NULL
until wimax_dev_add() is succesfully called. This allows calls into
the stack that happen before said time to be rejected.

Until now, the state was being set (by mistake) to UNINITIALIZED,
which was allowing calls such as wimax_report_rfkill_hw() to go
through even when a call to wimax_dev_add() had failed; that was
causing an oops when touching uninitialized data.

This situation is normal when the device starts reporting state before
the whole initialization has been completed. It just has to be dealt
with.

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