mtd: nand: fsmc: Fixes and cleanup for fsmc_nand_switch_ecc()

This patch addresses some comments raised by Scott in the last versions.
Here the changes in detail:

- Removed __maybe_unused as its not needed
- Added check for strength == 4 and error out for the unsupported
  ECC strength values
- Don't set .caclulate, .correct, and .bytes for NAND_ECC_SOFT_BCH as this
  will be done in nand_scan_tail()
- Set .caclulate back to fsmc_read_hwecc() in the HW case
- Added comment that this function will only be called on SPEAr platforms,
  not supporting the BCH8 HW ECC (FSMC_VER8)

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Tom Rini <trini@konsulko.com>
Acked-by: Scott Wood <scottwood@freescale.com>