ASoC: sgtl5000: fix cache handling

Cache handling in this driver is broken. The chip has 16-bit registers, yet the
register numbers also increase by 2 per register, i.e.  there are only
even-numbered registers. The cache in this driver, though, simply increments
register numbers, so it does need some mapping as seen in
sgtl5000_restore_regs(), note the '>> 1':

	snd_soc_write(codec, SGTL5000_CHIP_LINREG_CTRL,
                        cache[SGTL5000_CHIP_LINREG_CTRL >> 1]);

That, of course, won't work with snd_soc_update_bits(). (Thus, we won't even
notice the missing register 0x1c in the default regs which shifted all follwing
registers to wrong values.) Noticed on the MX28EVK where enabling the regulators
simply locked up the chip.

Refactor the routines and use a properly sized default_regs array which matches
the register layout of the underlying chip, i.e. create a truly flat cache.
This also saves some code which should make up for the bigger array a little.
When soc-core will somewhen have another cache type which handles a step size,
this conversion will also ease the transition.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Dong Aisheng <b29396@freescale.com>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org