r6040: bump release number to 0.18

This patch bumps the release of the r6040 driver. There has been
quite some versions of it out there, but this one is the one
people should report bugs against.

Signed-off-by: Florian Fainelli <florian.fainelli@telecomint.eu>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>