ALSA: Fix a const to non-const assignment in the Digigram VXpocket sound driver

Fix a const to non-const pointer assignment warning in the Digigram VXpocket
sound driver.

This may be due to patch 0aa4937648b91e9e6d3879b2cbeaa5f0c9863ac0.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>