net: wwan: core: split port creation and registration

net: wwan: core: split port creation and registration

Upcoming GNSS (NMEA) port type support requires exporting it via the
GNSS subsystem. On another hand, we still need to do basic WWAN core
work: find or allocate the WWAN device, make it the port parent, etc. To
reuse as much code as possible, split the port creation function into
the registration of a regular WWAN port device, and basic port struct
initialization.

To be able to use put_device() uniformly, break the device_register()
call into device_initialize() and device_add() and call device
initialization earlier.

While at it, fix a minor number leak upon WWAN port registration
failure.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Link: https://patch.msgid.link/20260126062158.308598-4-slark_xiao@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>