ARM: imx: fix i.MX6SLL build

The i.MX6SLL cpuidle support reuses the i.MX6SX implementation, but
the Makefile accidentally enables the i.MX6SL one as well, which
then fails with a link error unless the kernel also enables the
the i.MX6SL clock driver:

arch/arm/mach-imx/cpuidle-imx6sl.o: In function `imx6sl_enter_wait':
cpuidle-imx6sl.c:(.text+0x24): undefined reference to `imx6sl_set_wait_clk'

This changes the two lines that were just modified again, hopefully
getting every case right this time.

Fixes: e7fa1fb3 ("ARM: imx: add cpu idle support for i.MX6SLL")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>