[build] Remove unnecessary constraint on DBG_ENABLE()/DBG_DISABLE()

[build] Remove unnecessary constraint on DBG_ENABLE()/DBG_DISABLE()

DBG_ENABLE() and DBG_DISABLE() are currently constrained to enabling
and disabling only debug levels that are compiled in for the current
object.  For example, a DBG_ENABLE(DBGLVL_EXTRA) in foo.c will not be
able to affect output from other objects at DBGLVL_EXTRA unless foo.c
is itself compiled with DBGLVL_EXTRA enabled.

Partially fix by removing this unnecessary constraint.  (Note that it
is still necessary for at least one debug level to be compiled in for
the object invoking DBG_ENABLE()/DBG_DISABLE().)

Signed-off-by: Michael Brown <mcb30@ipxe.org>