Fix constant conversion warning in cxgb(4)

[dim]

Summary

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. While this should not effect the eventual
bits in the destination (which still end up as 0x80), it is better to
use uint8_t instead.