s390: fix clang -Wpointer-sign warnigns in boot code

The arch/s390/boot directory is built with its own set of compiler
options that does not include -Wno-pointer-sign like the rest of
the kernel does, this causes a lot of harmless but correct warnings
when building with clang.

For the atomics, we can add type casts to avoid the warnings, for
everything else the easiest way is to slightly adapt the types
to be more consistent.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/boot/ipl_parm.c

@@ -56,7 +56,7 @@ void store_ipl_parmblock(void)
 		ipl_block_valid = 1;
 }
 
-static size_t scpdata_length(const char *buf, size_t count)
+static size_t scpdata_length(const u8 *buf, size_t count)
 {
 	while (count) {
 		if (buf[count - 1] != '\0' && buf[count - 1] != ' ')