staging: rt2860: off by one errors

The code is trying to say that if the offset is higher than the max it
should be set to the max, but there is an off by one bug and it sets it
one passed the end of the array.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>