icE1usb fw: Fix computation of received E1 frames from USB

Several issues :

 - The reported length includes the CRC so it's minus 6 and
   not minus 4. (2 for CRC, 4 for header)

 - Cast length to int to make it signed so the minus works

 - If the packet is empty, this would be negative (no header),
   underflow, and then try to submit a giant number of frames
   to the E1 hardware

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Change-Id: Ib754e460290fe2e1551a0090e30a51846131d07d
1 file changed