ALSA: scarlett2: Remove repeated elem->head.mixer references

Use a local variable *mixer rather than repeating elem->header.mixer
in scarlett2_direct_monitor_ctl_get() and scarlett2_meter_ctl_get().

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/b21bacf4056366e10e01077e224d2b4970fdfe31.1703612638.git.g@b4.vu