Use the size of the input file as maximum tffs size instead of a fixed
value. The tffs on a AVM Fritz 300E can be up to 512KByte for example.
Fixes a read error for the AVM Fritz 3370 where the tffs partition size
is 64Kbyte and smaller than the former default value of 256KByte.
Signed-off-by: Mathias Kresin <dev@kresin.me>