loadb: Properly indicate aborted kermit transfer

When k_recv() returns zero it indicates that kermit transfer was aborted.
Function do_load_serial_bin() (caller of load_serial_bin()) interprets
value ~0 as aborted transfer, so properly propagates information about
aborted transfer from k_recv() to do_load_serial_bin().

Signed-off-by: Pali Rohár <pali@kernel.org>