If a second bootstream was used, there was a sanity check which was not
only wrong (using erasesize instead of writesize) but also superfluous
(we got the block from an existing cdev, so it must be in the flash
range). Simply remove it to make bcb work as expected.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>