bluetooth: rfcomm: fix the coverity 203443 memory corruptions

- Coverity think it is out-of-bound writing
because the codes write two bytes to address (&hdr->length)
that hdr->length is uint8_t. Coverity doesn't think about
the one more byte after the hdr->length.
- Use net_buf_push to resolve this issue.

Signed-off-by: Mark Wang <yichang.wang@nxp.com>
1 file changed