osd: fix signed char versus %02x issue

If char is signed and one of these bytes happen to have a value outside
the ascii range, the corresponding output will consist of "ffffff"
followed by the two hex chars that were actually intended. One way to
fix it would be to change the casts to (u8*) aka
(unsigned char*), but it is much simpler (and generates smaller code)
to use the %ph extension which was created for such short hexdumps.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Boaz Harrosh <ooo@electrozaur.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>