sf: always read 5 bytes for the idcode

Some SPI flash drivers like to have extended id information available
(like the spansion flash), so rather than making it re-issue the ID cmd
to get at the last 2 bytes, have the common code read 5 bytes rather than
just 3.  This also matches the Linux behavior where it always reads 5 id
bytes from all flashes.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
CC: Mingkai Hu <Mingkai.hu@freescale.com>