usb: misc: ftdi-elan: Fix off-by-one memory corruptions

This patch fixes fives off-by-one bugs in the ftdi-elan driver code. The
bug can be triggered by plugging a USB adapter for CardBus 3G cards (model
U132 manufactured by Elan Digital Systems, Ltd), causing a kernel panic.
The fix was tested on Ubuntu 14.04.4 with 4.7.0-rc14.2.0-27-generic+ and
4.4.0-22-generic+ kernel. In the ftdi_elan_synchronize function, an
off-by-one memory corruption occurs when packet_bytes is equal or bigger
than m. After having read m bytes, that is bytes_read is equal to m, "
..\x00" is still copied to the stack variable causing an out bounds write
of 4 bytes, which overwrites the stack canary and results in a kernel
panic.

This off-by-one requires physical access to the machine. It is not
exploitable since we have no control on the overwritten data.  Similar
off-by-one bugs have been observed in 4 other functions:
ftdi_elan_stuck_waiting, ftdi_elan_read, ftdi_elan_edset_output and
ftdi_elan_flush_input_fifo.

Reported-by: Alex Palesandro <palexster@gmail.com>
Signed-off-by: Xiao Han <xiao.han@orange.fr>
Tested-by: Paul Chaignon <pchaigno@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>