EDAC/mc: Fix grain_bits calculation

The grain in EDAC is defined as "minimum granularity for an error
report, in bytes". The following calculation of the grain_bits in
edac_mc is wrong:

	grain_bits = fls_long(e->grain) + 1;

Where grain_bits is defined as:

	grain = 1 << grain_bits

Example:

	grain = 8	# 64 bit (8 bytes)
	grain_bits = fls_long(8) + 1
	grain_bits = 4 + 1 = 5

	grain = 1 << grain_bits
	grain = 1 << 5 = 32

Replace it with the correct calculation:

	grain_bits = fls_long(e->grain - 1);

The example gives now:

	grain_bits = fls_long(8 - 1)
	grain_bits = fls_long(7)
	grain_bits = 3

	grain = 1 << 3 = 8

Also, check if the hardware reports a reasonable grain != 0 and fallback
with a warning to 1 byte granularity otherwise.

 [ bp: massage a bit. ]

Signed-off-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20190624150758.6695-2-rrichter@marvell.com