debian: Fix treatment of upstream version that contains hyphens.

debian: Fix treatment of upstream version that contains hyphens.

The Debian Policy Manual
(https://www.debian.org/doc/debian-policy/ch-controlfields.html#s-f-Version)
says that the upstream_version may contain only alphanumerics and the
characters . + - : ~ (full stop, plus, hyphen, colon, tilde) and should
start with a digit.

Currently, the upstream_version is defined in the debian/rules file:

DEB_UPSTREAM_VERSION=$(shell dpkg-parsechangelog | sed -rne 's,^Version: ([0-9]:)*([^-]+).*,\2,p')

The version number is taken from the dpkg-parsechangelog printout then the
first part of the version number which does not contain hyphen is filtered
out with sed. However the Debian Policy Manual says that hyphen is allowed
in the upstream_version.

This is not a problem with current vanilla OVS debian version. But, if a
postfix string including a hyphen is added to the upstream_version then
installation of datapath-dkms package will fail.

Reported-by: Zoltán Balogh <zoltan.balogh@ericsson.com>
Tested-by: Zoltán Balogh <zoltan.balogh@ericsson.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>