segfault due to buffer overrun in usb-serial

This fixes a segfault due to buffer overrun in the usb-serial device.
The memcpy was incrementing the start location by recv_used yet, the
computation of first_size (how much to write at the end of the buffer
before wrapping to the front) was not accounting for it. This causes the
next element after the receive buffer (recv_ptr) to get overwritten with
random data.

Signed-off-by: David Ahern <daahern@cisco.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
