Allow regular off-the-shelf keyboards to be overclocked like so-called
"Gamers Keyboards" by adding a tunable, "hw.usb.ukbd.pollrate", which
can fix the polling rate of the attached USB keyboards in the range
1..1000Hz. A similar feature already exists in the USB mouse
driver. Use with care! Might leave you without keyboard input. This
feature is only available when the USB_DEBUG option is set in the
kernel configuration file.

Correct "unit" type to "int" while at it.