Input: hyperv-keyboard - remove dependencies on PAGE_SIZE for ring buffer

Define the ring buffer size as a constant expression because it should
not depend on the guest page size.

Signed-off-by: Maya Nakamura <m.maya.nakamura@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>