rt2x00: unterminated strlen of user data

The buffer needs to be zero terminated in case the user data is not.
Otherwise we run off the end of the buffer.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>