ALSA: xen-front: fix unintention integer overflow on left shifts

Shifting the integer value 1 is evaluated using 32-bit
arithmetic and then used in an expression that expects a 64-bit
value, so there is potentially an integer overflow. Fix this
by using the BIT_ULL macro to perform the shift.

[ Note: as of the time being, no actual integer overflow hits because
  all values are less than 32bit, not including the extended 3-byte or
  DSD formats.  But this is the right fix for future usage, of
  course. -- tiwai ]

Addresses-Coverity: ("Unintentional integer overflow")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>