net: qrtr: ethernet: Fix incorrect buffer offset

While copying more bytes in a partial packet, the buffer pointer was
incorrectly shifted. So, only the last part of the packet was posted
to the qrtr core, resulting in invalid packet error.

Change-Id: I9823ea5cec1befa55917a0450adacb8e7233d94d
Signed-off-by: Jay Jayanna <jayanna@codeaurora.org>