Fix a const pointer usage warning in the Digigram miXart soundcard driver

Fix a const pointer usage warning in the Digigram miXart soundcard driver.  A
const pointer is being passed to copy_from_user() to load the firmware into.
This is okay in this case because the function has allocated the firmware
struct itself, but the const qualifier is part of the firmware struct - so the
patch casts the const away.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>