This patch fixes situation when one would like to write large file into
medium with the file system (fat, ext4, etc).
This change sets file size limitation to the DFU internal buffer size.

Since u-boot is not supporting interrupts and seek on file systems, it
becomes challenging to store large file appropriately.

To reproduce this error - create large file (around 26 MiB) and sent it
to the target board.

Lets examine the flow of USB transactions:

0. DFU uses EP0 with 64B MPS [Max Packet Size]

1. Send file - OUT (PC->target) - dat_26MiB.img is sent with 4096 B transactions

2. Get status - OUT (PC->target) - wait for DFU_STATE_dfuDNLOAD_IDLE (0x05) sent
				   from target board - IN transaction
				   (target->PC)

3. The whole file content is sent to target - OUT (PC->target) with ZLP [Zero
					      Length Packet]

Now the interesting part starts:

4. OUT (PC->target) Setup transaction (request to share DFU state)

5. IN (target->PC) - rep...