USB: sierra: remove incorrect usage of the urb status field

You can't rely on the fact that the status really is correct like it was.

Also simplified the write path and now we allocate the urb and data on
the fly, instead of trying to do that really odd timeout check which I
am guessing doesn't really work properly.  This should speed up the
device by keeping the hardware queue full easier.

As a benefit, this reduces the size of the driver.

Cc: Kevin Lloyd <linux@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>