Dell E7xx laptops have also mic mute LED that is driven by the
dell-laptop platform driver.  Bind it with the capture control as
already done for other models.

A caveat is that the fixup hook for the mic mute LED has to be applied
at last, otherwise it results in the invalid override of the callback.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=205529
Link: https://lore.kernel.org/r/20200105081119.21396-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>