sos: don't change NBITS.

For SOS, an ECC point with prefix 0x40 can be represented in two ways.
One for 0x40 interpreted as 7-bit, another as 8-bit (MSB is zero).

This matters for fingerprint computation, so, the representation
should be preserved, or we should use another flag to hold the
difference in the representation...

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>