NAND: davinci: choose correct 1-bit h/w ECC reg

NAND: davinci: choose correct 1-bit h/w ECC reg

In nand_davinci_readecc(), select the correct NANDF<n>ECC register based
on CONFIG_SYS_NAND_CS rather than hardcoding the choice of NANDF1ECC.
This allows 1-bit hardware ECC to work with chip select other than CS2.

Note this now matches the usage in nand_davinci_enable_hwecc(), which
already had the correct handling, and allows refactoring to a single
function encapsulating the register read.

Without this fix, writing NAND pages to a chip not wired to CS2 would
result in in the ECC calculation always returning FFFFFF for each
512-byte segment, and reading back a correctly written page (one with
ECC intact) would always fail. With this fix, the ECC is written and
verified correctly.

Signed-off-by: Laurence Withers <lwithers@guralp.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>