Change a static const string to a #define as the strcpy() throws a
warn/error with clang.

/home/sbruno/mips64-clang/sys/mips/rmi/xls_ehci.c:133:25: error:
format string is not a string literal (potentially insecure)
[-Werror,-Wformat-security]

sprintf(sc->sc_vendor, xlr_vendor_desc);