metag: cachepart: Fix failure check

[linux-3.16-rc5/arch/metag/kernel/cachepart.c:102]: (style) Checking if
unsigned variable 'thread_cache_size' is less than zero.

Source code is

        if (thread_cache_size < 0)
            pr_emerg("Can't read %s cache size\n",
                 cache_type ? "DCACHE" : "ICACHE");

but

    unsigned int thread_cache_size;

Function get_thread_cache_size returns an error code
as (unsigned int) -1.

Change get_thread_cache_size() to return signed int, and its result is
stored into signed int variable.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=80361


Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Signed-off-by: James Hogan <james.hogan@imgtec.com>