usb: gadget: u_serial: reimplement console support

Rewrite console support to fix a few shortcomings of the old code preventing its use with multiple ports. This removes some duplicated code and replaces a custom kthread with simpler workqueue item.

Only port ttyGS0 gets to be a console for now.

Signed-off-by Michał Mirosław <[email protected]>

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 drivers/usb/gadget/function/u_serial.c | 352 ++++++++++++-------------
 1 file changed, 164 insertions(+), 188 deletions(-)