Don't use CHECK_VOL(x) as a loop conditional
...On non-multidrive targets it will always evaluate to true because
it effectively ignores the argument passed in.
Instead use 'x < NUM_VOLUMES'
Fixes regression introduced with a41a001258 on non-multivolume devices
Change-Id: I1a620af113c87cf3227ef8b40d0b51b2600cd816