s390/time: Fix clk type in get_tod_clock

Clang warns:

In file included from ../arch/s390/boot/startup.c:3:
In file included from ../include/linux/elf.h:5:
In file included from ../arch/s390/include/asm/elf.h:132:
In file included from ../include/linux/compat.h:10:
In file included from ../include/linux/time.h:74:
In file included from ../include/linux/time32.h:13:
In file included from ../include/linux/timex.h:65:
../arch/s390/include/asm/timex.h:160:20: warning: passing 'unsigned char
[16]' to parameter of type 'char *' converts between pointers to integer
types with different sign [-Wpointer-sign]
        get_tod_clock_ext(clk);
                          ^~~
../arch/s390/include/asm/timex.h:149:44: note: passing argument to
parameter 'clk' here
static inline void get_tod_clock_ext(char *clk)
                                           ^

Change clk's type to just be char so that it matches what happens in
get_tod_clock_ext.

Fixes: 57b28f66 ("[S390] s390_hypfs: Add new attributes")
Link: https://github.com/ClangBuiltLinux/linux/issues/861
Link: http://lkml.kernel.org/r/20200208140858.47970-1-natechancellor@gmail.comReviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/timex.h

@@ -155,7 +155,7 @@ static inline void get_tod_clock_ext(char *clk)
 
 static inline unsigned long long get_tod_clock(void)
 {
-	unsigned char clk[STORE_CLOCK_EXT_SIZE];
+	char clk[STORE_CLOCK_EXT_SIZE];
 
 	get_tod_clock_ext(clk);
 	return *((unsigned long long *)&clk[1]);