ldlinux: Use signed char consistently

ldlinux: Use signed char consistently

In get_key() we use unsigned chars but seem to use signed chars in
other functions. There's no real reason to use unsigned chars so let's
be consistent. This also eliminates the following warnings,

get_key.c: In function ‘get_key’:
get_key.c:187:2: warning: pointer targets in passing argument 1 of ‘get_key_decode’ differ in signedness
get_key.c:129:5: note: expected ‘char *’ but argument is of type ‘unsigned char *’
serirq.c:29:29: warning: pointer targets in initialization differ in signedness
serirq.c:30:29: warning: pointer targets in initialization differ in signedness

Signed-off-by: Matt Fleming <matt.fleming@intel.com>