microblaze: Fix strncpy_from_user macro

Problem happens when len in strncpy_from_user is setup
and passing string has len-1 chars + \0 terminated
character. In this case was returned incorrect length
of the string.
It should always retunrs the length of the string
(not including the trailing NULL).

Signed-off-by: Michal Simek <michal.simek@xilinx.com>