CFI: fix eraseregions numblocks

eraseregions numblocks was sometimes one less than actual, possibly producing
erase regions with zero blocks. As MTD code touches eraseregions only if
numeraseregions is greater that zero, allocate eraseregions only for non
uniform erase size flash.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Stefan Roese <sr@denx.de>