ASoC: skl: Fix kernel warning due to zero NHTL entry

I got the following kernel warning when loading snd-soc-skl module on
Dell Latitude 7270 laptop:
 memremap attempted on mixed range 0x0000000000000000 size: 0x0
 WARNING: CPU: 0 PID: 484 at kernel/memremap.c:98 memremap+0x8a/0x180
 Call Trace:
  skl_nhlt_init+0x82/0xf0 [snd_soc_skl]
  skl_probe+0x2ee/0x7c0 [snd_soc_skl]
  ....

It seems that the machine doesn't support the SKL DSP gives the empty
NHLT entry, and it triggers the warning.  For avoiding it, let do the
zero check before calling memremap().

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>