dfu: Make maximum DFU file size equal to default DFU data buffer

dfu: Make maximum DFU file size equal to default DFU data buffer

Up till now the DFU maximum file size (to be written to e.g. eMMC)
was different from the DFU data buffer size. It caused errors when
one buffer was smaller than data to be written.

Now, the maximum DFU file size is equal to default DFU buffer size.
In spite of this, user is still able to manually adjust those default
values.

Change-Id: Ied75d0f7b59588ebd79dae9a22af801d36622216
Signed-off-by: Lukasz Majewski <[email protected]>