mfd: Remove build time warning for WM835x register default tables

Systems using the WM835x need to choose which of the default register
settings are required on the system.  Currently there is a compile time
warning as well as a runtime error intended to flag up to users that
this is required but this also triggers for people building the driver
in order to obtain build coverage.

Remove the build warning, leaving only the runtime error, in order to
reduce noise for people doing generic kernel work.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>