mips: cache: Bulletproof the code against cornercases

This patch makes sure that the flush/invalidate_dcache_range() functions
can handle corner-case calls like this -- invalidate_dcache_range(0, 0, 0);
This call is valid and is happily produced by USB EHCI code for example.
The expected behavior of the cache function(s) in this case is that they
will do no operation, since the size is zero.

The current implementation though has a problem where such invocation will
result in a hard CPU hang. This is because under such conditions, where the
start_addr = 0 and stop = 0, the addr = 0 and aend = 0xffffffe0 . The loop
will then try to iterate over the entire address space, which in itself is
wrong. But iterating over the entire address space might also hit some odd
address which will cause bus hang. The later happens on the Atheros MIPS.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Hans de Goede <hdegoede@redhat.com>

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