powerpc/perf: Remove PPMU_HAS_SSLOT flag for Power8

Commit 7a786832 ("powerpc/perf: Add an explict flag indicating
presence of SLOT field") introduced the PPMU_HAS_SSLOT flag to remove
the assumption that MMCRA[SLOT] was present when PPMU_ALT_SIPR was not
set.

That commit's changelog also mentions that Power8 does not support
MMCRA[SLOT]. However when the Power8 PMU support was merged, it
errnoeously included the PPMU_HAS_SSLOT flag.

So remove PPMU_HAS_SSLOT from the Power8 flags.

mpe: On systems where MMCRA[SLOT] exists, the field occupies bits 37:39
(IBM numbering). On Power8 bit 37 is reserved, and 38:39 overlap with
the high bits of the Threshold Event Counter Mantissa. I am not aware of
any published events which use the threshold counting mechanism, which
would cause the mantissa bits to be set. So in practice this bug is
unlikely to trigger.

Fixes: e05b9b9e ("powerpc/perf: Power8 PMU support")
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>