exfat: debian: comply to debian policy for upstream_version

From 5.6.12:
"If there is no debian_revision then hyphens are not allowed."

Also use the debhelper variables for getting upstream_version.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Park Ju Hyung <qkrwngud825@gmail.com>