usb: serial: baseband: buffer allocations at init

usb: serial: baseband: buffer allocations at init

The usb transaction buffers are allocated in module init.
In device open, close and disconnect the buffers will not be
freed. Instead they will be reused to avoid allocation failure
in low-mem conditions.

The usb driver register moved to init so that rmmod and
insmod is not required.

Bug 956211

Signed-off-by: Vinayak Pane <vpane@nvidia.com>
Reviewed-on: http://git-master/r/109935
(cherry picked from commit a4e8219a86f80fd06aaaae2c40a657098d5dcfa5)

Change-Id: Id88cfe3b0a75cb2e6f39176b5297f81f4f9e978b
Reviewed-on: http://git-master/r/115609
Tested-by: Vinayak Pane <vpane@nvidia.com>
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Steve Lin <stlin@nvidia.com>
Rebase-Id: R1fe1602c604a4339bd845677cd87db4966a1ff31