umbim: detect actual connection IP type

umbim: detect actual connection IP type

Current implementation needlessly creates both IPv4 and IPv6
sub-interfaces for single-stack IP types. Limit this only to selected IP
type. While at that, ensure that IP type is also passed to umbim during
"connect" phase. In addition, detect the actual established connection
type returned by umbim and set up subinterfaces according to that,
not to requested configuration. While at that, allow empty IP type explicitly,
interpreted as "any" according to MBIM specification.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>