sound: bcm: Fix memset dereference warning

This warning appears with GCC 6.4.0 from toolchains.bootlin.com:

../sound/soc/bcm/allo-piano-dac-plus.c: In function ‘snd_allo_piano_dac_init’:
../sound/soc/bcm/allo-piano-dac-plus.c:711:30: warning: argument to ‘sizeof’ in ‘memset’ call is the same expression as the destination; did you mean to dereference it? [-Wsizeof-pointer-memaccess]
  memset(glb_ptr, 0x00, sizeof(glb_ptr));
                              ^

Suggested-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>