ASoC: davinci-mcasp: Do not touch 0x04 register above McASP_VERSION_2

This register is not defined in TI81xx and on AM335x/AM437x it is the
SYSCONFIG register which should not be touched by drivers since it is
related to PM and handled by the generic PM code.
This register write was there since the first time the davinci-mcasp driver
was appeared in the kernel.
The reason why it did not caused any issues on AM335x/AM437x is that it sets
bit 1 in SYSCONFIG register which in turn will enable the smart-idle mode.
This is the default mode and this is the mode McASP should be in also when
in use.
On TI81xx the register is not defined.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>