MFC r305360:

MFC r305360:

With clang 3.9.0, compiling cxgb results in the following warning:

sys/dev/cxgb/cxgb_sge.c:2873:44: error: implicit conversion from 'int'
to 'char' changes value from 128 to -128 [-Werror,-Wconstant-conversion]
                        *mtod(m, char *) = CPL_ASYNC_NOTIF;
                                         ~ ^~~~~~~~~~~~~~~

This is because CPL_ASYNC_NOTIF is 0x80, so the plain char argument is
wrapped to a negative value.  Fix this by using uint8_t instead.

Reviewed by:	np
Differential Revision: https://reviews.freebsd.org/D7772

git-svn-id: svn://svn.freebsd.org/base/stable/9@305556 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f