mtd: nand: add sanity check of ecc strength to nand_scan_tail()

mtd: nand: add sanity check of ecc strength to nand_scan_tail()

This patch adds sanity checks that ensure that drivers for controllers with
hardware ECC set the 'strength' element in struct nand_ecc_ctrl.  Also stylistic
changes to the line that calculates strength for software ECC.

This v2 simplifies the check.  Thanks Brian! [1]

[1] http://lists.infradead.org/pipermail/linux-mtd/2012-April/040890.html

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>