shell: logging: Fix assertion when shell is overflowed with logging

When shell had too many pending log messages it was attempting to
drop expired messages and retrying to put the new message. There
was an assumption that enough messages are dropped and new message
can be put. It may not be the case if no message expired during
given time. Wrapped the operation in loop to continue until expired
message is freed and new message is enqueued.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>