migration: Use migrate_mode() to query for cpr-transfer

cpr_set_incoming_mode() is only called on the target side, so
migrate_mode() on the source side is the same as s->parameters.mode.

Use the function to reduce explicit access to s->parameters, we have
options.c for that.

Cc: Mark Kanda <mark.kanda@oracle.com>
Cc: Ben Chaney <bchaney@akamai.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Link: https://lore.kernel.org/qemu-devel/20260123141656.6765-7-farosas@suse.de
Signed-off-by: Fabiano Rosas <farosas@suse.de>