IB/mthca: Always build debugging code unless CONFIG_EMBEDDED=y

Change the mthca debugging trace output code so that it can enabled
and disabled at runtime with the debug_level module parameter in
sysfs.  Also, don't allow CONFIG_INFINIBAND_MTHCA_DEBUG to be disabled
unless CONFIG_EMBEDDED is selected.  We want users (and especially
distros) to have this turned on unless they really need to save space,
because by the time we want debugging output, it's usually too late to
rebuild a kernel.

Signed-off-by: Roland Dreier <rolandd@cisco.com>