metag: cachepart: fix get_global_dcache_size() typo

Compilation is broken when the kernel is destined to live in the global
part of the virtual address space:

arch/metag/kernel/cachepart.c In function 'get_thread_cache_size':
arch/metag/kernel/cachepart.c +71 : error: implicit declaration of function 'get_global_dache_size'

Fix the typo.

Signed-off-by: James Hogan <james.hogan@imgtec.com>