dfu: Fix write block size check on, probably, incorrect device

The write block size has been gathered from device pointed by
chosen 'zephyr,flash', while it should be taken from a device
a target image flash area resides on.
The write block size has been used to check if write buffer
is properly aligned.
The check is only possible on devices that have write-block-size
property in DTS, so in case of SPI connected devices this property
may not be available - when device does not have the property,
the check is not performed.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>