arm: Simplify get_cachetype_cp15()

There's no point in setting 'arm_dcache_align_mask' before the
function's end.

Reviewed by: markj, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42261

(cherry picked from commit 1bce6f951a902f03bfb354f5b11473a0d12b3d7d)