markers: fix sparse warnings in markers.c

char can be unsigned
kernel/marker.c:64:20: error: dubious one-bit signed bitfield
kernel/marker.c:65:14: error: dubious one-bit signed bitfield

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>