[usb] reduce USB MaxPower to 100 mA (#579)

The USB transport currently defines MaxPower as 500mA. With some
bus-powered USB HUBs, the host might refuse enumeration due to limited
available power.

This commit reduce MaxPower to 100 mA, which corresponds to the maximum
current allowed to be used without enumeration. According to the NRF52
datasheets, this is still overly conservative, even summing the maximum
CPU and peripherals (including radio) consumption.

third_party/NordicSemiconductor/config/app_config.h

@@ -78,7 +78,7 @@
 #endif
 
 #ifndef APP_USBD_CONFIG_MAX_POWER
-#define APP_USBD_CONFIG_MAX_POWER 500
+#define APP_USBD_CONFIG_MAX_POWER 100
 #endif
 
 #ifndef APP_USBD_STRING_ID_PRODUCT