include MM_PLUGIN_ALLOWED_SINGLE_AT in port probing
As discussed in this issue ,if a plugin only specifies MM_PLUGIN_ALLOWED_SINGLE_AT
as part of the plugin creation options then port probing will be skipped for the plugin. MM_PLUGIN_ALLOWED_SINGLE_AT
already implies an AT port available for probing so it should not be necessary to additionally specify MM_PLUGIN_ALLOWED_AT
.