powerpc/lib/sstep: Fix 'sthcx' instruction

Looks like there been a copy paste mistake when added the instruction
'stbcx' twice and one was probably meant to be 'sthcx'. Changing to
'sthcx' from 'stbcx'.

Fixes: 350779a2

 ("powerpc: Handle most loads and stores in instruction emulation code")
Cc: stable@vger.kernel.org # v4.14+
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220224162215.3406642-1-anders.roxell@linaro.org