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

Fix a const pointer usage warning in the Digigram pcxhr compatible 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>