Change CFG_ENV_SIZE to CFG_ENV_SECT_SIZE for SPI sector erase

The CFG_ENV_SIZE is not suitable used for SPI flash erase
sector size if CFG_ENV_SIZE is less than CFG_ENV_SECT_SIZE.
Add condition check if CFG_ENV_SIZE is larger than
CFG_ENV_SECT_SIZE, calculate the right number of sectors for
erasing.

Signed-off-by: TsiChung Liew <Tsi-Chung.Liew@freescale.com>