Given that most of drivers/usb/serial/Kconfig is wrapped inside:

  if USB_SERIAL
  ...
  endif # USB_SERIAL

remove the consequently redundant dependencies on USB_SERIAL.
Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>