cmsis-dap: don't update the packet size across backends.

The hidapi cmsis-dap backend is using a packet_size of
64+1: 64 is the bMaxPacketSize0 and 1 is the hid Report-Id.

In hidapi::hid_write(), the packet_size is decremented by 1 and
stored for the next transfer.
The packet_size is now valid bMaxPacketSize0=64,
so when hid_read() is called, libusb_bulk_transfer() finishes w/o timeout.

For the libusb bulk backend, the same packet_size of 64+1 is used,
but there's no hid_write() to decrement and store it for the next read.

So the next time a read is done, it will try to read 64+1 bytes.

Fix this by putting the packet logic within each backend.
Use calloc() to allocate the struct cmsis_dap to be on safer side.

Change-Id: I0c450adbc7674d5fcd8208dd23062d5cdd209efd
Signed-off-by: Adrian Negreanu <adrian.negreanu@nxp.com>
Reviewed-on: http://openocd.zylin.com/5920
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>