powerpc: PTRACE_PEEKUSR/PTRACE_POKEUSER of FPR registers in little endian builds

FPRs overlap the high 64bits of the first 32 VSX registers. The
ptrace FP read/write code assumes big endian ordering and grabs
the lowest 64 bits.

Fix this by using the TS_FPR macro which does the right thing.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>