xen/console: Fix build when CONFIG_DEBUG_TRACE=y

xen/console: Fix build when CONFIG_DEBUG_TRACE=y

Commit b5e6e1ee8da "xen/console: Don't treat NUL character as the end
of the buffer" extended sercon_puts to take the number of character
to print in argument.

Sadly, a couple of couple of the callers in debugtrace_dump_worker()
were not converted. This result to a build failure when enabling
CONFIG_DEBUG_TRACE.

Spotted by Travis using randconfig
Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>