configs/topic_miami.h: Correct kernel_size in default environment

The kernel partition in QSPI is 0x440000 large, not 0x400000. Fix this
in the environment, otherwise the kernel will fail to boot if it occupies
more space.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>