mgmt: mcumgr: transport: src: fix smp_udp_open() corrupting socket var

When smp_udp_open() was executed more than once, create_socket() returned
an error code (EALREADY) rather than a newly created socket FD. This error
was saved in a global variable that was then directly accessed in a thread
by recvfrom(), causing the loop in the thread to start spinning and
printing error messages uninterruptedly.

Fixes #58172

Signed-off-by: Marco Argiolas <marco.argiolas@ftpsolutions.com.au>