alpha: fix for no 'break' in the case statement

alpha: fix for no 'break' in the case statement

gem5 won't compile correctly since g++ will throw a warning (error)
that the next case statement below this one is reachable since there is
no 'break' statement.

Signed-off-by: Sujay Phadke
Change-Id: Icab646ee5abcfeb6ba3e690909042927b4003eba
Reviewed-on: https://gem5-review.googlesource.com/7521
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>