wilc1000: fix compiler warning for 'wowlan_support' unused variable

Avoid below reported warning found when 'CONFIG_PM' config is
undefined.

'warning: unused variable 'wowlan_support' [-Wunused-const-variable]'

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200710051826.3267-4-ajay.kathat@microchip.com