powerpc/time: Make get_tb() common to PPC32 and PPC64

mftbu() is always defined now, so the #ifdef can be removed
and replaced by an IS_ENABLED(CONFIG_PPC64) inside the
PPC32 version of get_tb().
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/47e49717d2643169ffcbe5d507f184cf49f0fe95.1601556145.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/time.h

@@ -53,16 +53,13 @@ static inline u64 get_vtb(void)
 	return 0;
 }
 
-#ifdef CONFIG_PPC64
-static inline u64 get_tb(void)
-{
-	return mftb();
-}
-#else /* CONFIG_PPC64 */
 static inline u64 get_tb(void)
 {
 	unsigned int tbhi, tblo, tbhi2;
 
+	if (IS_ENABLED(CONFIG_PPC64))
+		return mftb();
+
 	do {
 		tbhi = mftbu();
 		tblo = mftb();
@@ -71,7 +68,6 @@ static inline u64 get_tb(void)
 
 	return ((u64)tbhi << 32) | tblo;
 }
-#endif /* !CONFIG_PPC64 */
 
 static inline void set_tb(unsigned int upper, unsigned int lower)
 {