drivers/ftdi: free allocated memory

Also uses calloc() for mpsse_ctx->write_buffer to prevent a false
positive valgrind report
"Syscall param ioctl(USBDEVFS_SUBMITURB).buffer points to uninitialised bytes(s)"

Change-Id: I91963371d15c21ea0fee4c40c1da86174db44520
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4418
Tested-by: jenkins