ARM: pxa: correct errata number for PXA270

Comment incorrectly cites errata 39
    E39. SDIO: SDIO Devices Not Working at 19.5 Mbps

Should be errata 38
    E38. MEMC: Memory Controller hangs when entering Self Refresh Mode.

Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Acked-by: Haojian Zhuang <haojian.zhuang@linaro.org>