mtd: bcm47xxpart: lower minimal blocksize to 4Ki (from 64Ki)

Some devices like Netgear WNR1000v3 or WGR614v10 have partitions aligned
to 0x1000. Using bigger blocksize stopped us from detecting some parts.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>