USB: Remove superfluous "depends on USB_SERIAL" from Kconfig.

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: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>