mfd: Remove unused variable from da9052_device_init

This is to address the following warning during compilation time:

  drivers/mfd/da9052-core.c: In function ‘da9052_device_init’:
  drivers/mfd/da9052-core.c:646: warning: unused variable ‘desc’

This variable is indeed no longer in use (change can be traced back
to commit: 86144194).

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>