ftrace: fix hex output mode of ftrace

Fix the output of ftrace in hex mode as the hi/lo nibbles are output in
reverse order. Without this patch, the output of ftrace is:

raw mode : 6474 0 141531612444 0 140 + 6402 120 S
hex mode : 000091a4 00000000 000000023f1f50c1 00000000 c8 000000b2 00009120 87 ffff00c8 00000035

There is an inversion on ouput hex(6474) is 194a

[based on a patch by Philippe Reynes <tremyfr@yahoo.fr>]
Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>