mtd: mtdswap: use MTDSWAP_ECNT_MIN/MAX

Since macros MTDSWAP_ECNT_MIN() and MTDSWAP_ECNT_MAX() have been
defined in mtdswap.c, use them instead of open-coding.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>