fade: Quiesce two gcc warnings.

Ubuntu Lucid's gcc 4.4.3 does not like it if not each possible value
of an enumeration is handled in the body of a switch statement.

Since gengetopt for some reason always creates an additional xxx_NULL
value in options of type enum, this more or less means we have to
introduce default: cases to avoid warnings of the form

	fade.c:113: warning: enumeration value ‘mixer_channel__NULL’ not handled in switch
	fade.c:346: warning: enumeration value ‘mode__NULL’ not handled in switch

Fortunately, these are the only two warnings, so add default: cases
to make gcc happy.