modem: cmux: Do not return error on low buffer space

If we end up writing zero bytes to cmux output, we can return
zero instead of -ENOMEM as it would break various modules when
using small buffers. For example modem_chat.c does not tolerate
-ENOMEM but handles zero OK.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>