wacom: Use the ExpressKey Remote name for the EKR
There's only one device like this that we support so we might as well use its name as-is instead of a more confusing "External Pad device" which isn't very descriptive unless you work with tablets and know what a "pad" is in this context.
Anyone who owns this device will have bought it under the name ExpressKey remote.
Fixes #2044 (closed)
FTR, even wacom japan calls it ExpressKey Remote.
cc @Pinglinux, @jigpu