Rename struct bt_micp to bt_mic_ctlr as it is only
used ofr microphone controller now.
This also allow cleanup of the bt_micp struct, thus
removing the common internal micp_internal.h header file
and reducing the size of the implementation by not
using a union of the structs.
Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>