Input: e3x0-button - update Kconfig description

The driver is now used for the entire USRP e3xx series,
this commit fixes the description that will be displayed in
the menu accordingly.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>