common: command: Fix command auto-completion

When auto-completing command arguments, the last argument is not
necessarily the one we need to auto-complete. When the last character is
a space, a tab or '\0' what we want instead is list all possible values,
or if there's only one possible value, place this value on the command
line instead of trying to suffix the last valid argument with missing
chars.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Tom Rini <trini@konsulko.com>