bridgevlanport: replace sed pipe with substitution

bridgevlanport: replace sed pipe with substitution

The previous code stripped *all* combinations of zero or
more letters and digits followed by a dot of an interface
name to retrieve the VLANID. At the same time the supposedly
opposite code for IF_VLAN_RAW_DEVICE only actually extracts
the part up to the *first* dot. Thus an interface named
'a.b.0' would have a VLANID of 0 and a raw device name of
just 'a'.
I also don't see a reason to limit the removed portions to
alphanumeric characters. I.e. an interface named tap-test.0
would result in a VLANID of 'tap-0' and an
IF_VLAN_RAW_DEVICE of 'test'.

A simple shell substitution seems to do a much better job
and is more efficient than forking out two processes with a
pipe.