[PATCH] usb-serial: Fix data corruption with usb serial emulation

* Remove the unused send_buf variable and its constant.

* Fix a math error
  The variables recv_ptr and recv_used are not large enough to hold
  the constant 384, which causes data corruption when the pointer is
  reset with: s->recv_ptr = (s->recv_ptr + len) % RECV_BUF;

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5242 c046a42c-6fe2-441c-8c8c-71466251a162
