sh64: kernel: use 'usp' instead of 'fn'

'fn' is not defined, according to the implementation copy_thread() in
'sh32', need use 'usp' instead of.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>