hid: Replace USBHID_ENABLED kernel config option with loader tunable

usbhid(4) is disabled by default to avoid conflicts with existing USB HID
drivers. To enable it place following lines to /boot/loader.conf:

hw.usb.usbhid.enable=1
usbhid_load="YES"

Suggested by: jhb
Reviewed by: hselasky
Differential revision: https://reviews.freebsd.org/D28124