Fix cpsw(4) after r277203 which folded 'struct m_hdr' into 'struct mbuf'.
While in theory this should have been a transparent change (and was for all
other drivers), cpsw(4) never used the proper accessor macros in a few
places but spelt the indirect m_hdr.mh_* out itself. Convert those to
use m_len and m_data and unbreak the driver build.