x86: suppress trivial sparse signedness warnings

Could just as easily change the three casts to cast to the correct
type...this patch changes the type of ftrace_nop instead.

Supresses sparse warnings:

 arch/x86/kernel/ftrace.c:157:14: warning: incorrect type in assignment (different signedness)
 arch/x86/kernel/ftrace.c:157:14:    expected long *static [toplevel] ftrace_nop
 arch/x86/kernel/ftrace.c:157:14:    got unsigned long *<noident>
 arch/x86/kernel/ftrace.c:161:14: warning: incorrect type in assignment (different signedness)
 arch/x86/kernel/ftrace.c:161:14:    expected long *static [toplevel] ftrace_nop
 arch/x86/kernel/ftrace.c:161:14:    got unsigned long *<noident>
 arch/x86/kernel/ftrace.c:165:14: warning: incorrect type in assignment (different signedness)
 arch/x86/kernel/ftrace.c:165:14:    expected long *static [toplevel] ftrace_nop
 arch/x86/kernel/ftrace.c:165:14:    got unsigned long *<noident>

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>