drivers: console: fix dropped characters when using debug hooks

When using console debug server hooks, not all characters are
processed if the server hook returns non-zero for one character
while there are other characters in the buffer. This is seen
when using a fast console (like USB) where multiple characters
come in before the ISR is called. Fix it by continuing to
process characters instead of returning from the ISR with
characters still in the buffer.

Fixes: #64661

Signed-off-by: Mariano Goluboff <mariano.goluboff@nordicsemi.no>