ALSA: ctxfi: use enum type CT_SUM_CTL where appropriate

Currently a variable of type enum CT_AMIXER_CTL is used for enum
CT_SUM_CTL values. This leads to warnings when using clang:
sound/pci/ctxfi/ctmixer.c:945:32: warning: implicit conversion from enumeration type
      'enum CT_SUM_CTL' to different enumeration type 'enum CT_AMIXER_CTL'
      [-Wenum-conversion]
        for (i = AMIXER_MASTER_F, j = SUM_IN_F;
                                    ~ ^~~~~~~~
sound/pci/ctxfi/ctmixer.c:975:29: warning: implicit conversion from enumeration type
      'enum CT_SUM_CTL' to different enumeration type 'enum CT_AMIXER_CTL'
      [-Wenum-conversion]
        for (i = AMIXER_PCM_F, j = SUM_IN_F; i <= AMIXER_PCM_S; i++, j++) {
                                 ~ ^~~~~~~~

Introduce enum CT_SUM_CTL k and it instead.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Takashi Iwai <tiwai@suse.de>