drm/nouveau: fix i2ctable bounds checking

i2c_entries seems to be the number of i2c entries,
so with index equal to this number, we could read
invalid data from i2ctable. Fix it.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>