powerpc/configs/6xx: Clean up duplicate CONFIG_EXT4 values

We had two values for CONFIG_EXT4, =m and =y, just use =y.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/configs/ppc6xx_defconfig

@@ -1022,10 +1022,9 @@ CONFIG_EXT2_FS=m
 CONFIG_EXT2_FS_XATTR=y
 CONFIG_EXT2_FS_POSIX_ACL=y
 CONFIG_EXT2_FS_SECURITY=y
-CONFIG_EXT4_FS=m
+CONFIG_EXT4_FS=y
 CONFIG_EXT4_FS_POSIX_ACL=y
 CONFIG_EXT4_FS_SECURITY=y
-CONFIG_EXT4_FS=y
 CONFIG_JBD2_DEBUG=y
 CONFIG_REISERFS_FS=m
 CONFIG_REISERFS_PROC_INFO=y