jtag/vsllink: switch to command 'adapter serial'

jtag/vsllink: switch to command 'adapter serial'

The driver vsllink defines the command 'vsllink usb_serial' to
specify the serial string of the adapter.

Remove and deprecate the driver command, and use 'adapter serial'.

Change-Id: Iadcc018b8aa8974ccd7156915b84e58270fad29d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6654
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins