[ALSA] sound: ali5451.c fix shadowed variable warnings

enable is used to test for whether or not spdif should be enabled,
change to spdif_enable.

sound/pci/ali5451/ali5451.c:1812:15: warning: symbol 'enable' shadows an earlier one
sound/pci/ali5451/ali5451.c:63:12: originally declared here
sound/pci/ali5451/ali5451.c:1840:27: warning: symbol 'enable' shadows an earlier one
sound/pci/ali5451/ali5451.c:63:12: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>