Fixed unused-function warning
Fixed unused-function warning

See: https://github.com/esp8266/Arduino/pull/2881/files/d55efd3e144256afe043d251a0f6c778c5139f7c#diff-67390c99ad4faa2080719586ebf73b9e and https://travis-ci.org/felis/USB_Host_Shield_2.0/jobs/213822247#L1248

lrmoreno007/Arduino,Juppit/Arduino,Juppit/Arduino,me-no-dev/Arduino,Adam5Wu/Arduino,hallard/Arduino,wemos/Arduino,lrmoreno007/Arduino,Lan-Hekary/Arduino,me-no-dev/Arduino,Juppit/Arduino,Lan-Hekary/Arduino,Lan-Hekary/Arduino,sticilface/Arduino,sticilface/Arduino,Juppit/Arduino,hallard/Arduino,esp8266/Arduino,lrmoreno007/Arduino,wemos/Arduino,Adam5Wu/Arduino,Lan-Hekary/Arduino,wemos/Arduino,sticilface/Arduino,sticilface/Arduino,esp8266/Arduino,sticilface/Arduino,lrmoreno007/Arduino,jes/Arduino,Adam5Wu/Arduino,jes/Arduino,hallard/Arduino,esp8266/Arduino,lrmoreno007/Arduino,Lan-Hekary/Arduino,hallard/Arduino,jes/Arduino,Juppit/Arduino,wemos/Arduino,me-no-dev/Arduino,wemos/Arduino,hallard/Arduino,Adam5Wu/Arduino,Adam5Wu/Arduino,me-no-dev/Arduino,me-no-dev/Arduino,jes/Arduino,esp8266/Arduino,esp8266/Arduino,jes/Arduino