ASoC: wm8753: fix initialization

Without that fix the wm8753 SPI initialization fails, and then produces
  a kernel panic during boot with the following call trace:
    Unable to handle kernel paging request at virtual address 37386d9b
    [<c01ccafc>] (regmap_get_val_bytes+0x0/0x14) from [<c0243dfc>] (snd_soc_codec_set_cache_io+0x9c/0xcc)
    [<c0243dfc>] (snd_soc_codec_set_cache_io+0x9c/0xcc) from [<c0244a4c>] (wm8753_probe+0x5c/0x1c4)
    [<c0244a4c>] (wm8753_probe+0x5c/0x1c4) from [<c023bb24>] (soc_probe_codec+0x174/0x284)
    [<c023bb24>] (soc_probe_codec+0x174/0x284) from [<c023c2c0>] (snd_soc_instantiate_cards+0x68c/0xe28)
    [<c023c2c0>] (snd_soc_instantiate_cards+0x68c/0xe28) from [<c023d278>] (snd_soc_register_card+0x240/0x2d4)
    [<c023d278>] (snd_soc_register_card+0x240/0x2d4) from [<c023d330>] (soc_probe+0x24/0x40)
    [<c023d330>] (soc_probe+0x24/0x40) from [<c01c3900>] (platform_drv_probe+0x14/0x18)
    [...]

The commit d3398ff0
 ( ASoC: Convert WM8753 to direct regmap API usage ) introduced
 the problem.

Thanks to Lars-Peter Clausen for helping me a bit during the debugging.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>