USB: serial: keyspan_pda: clean up xircom/entrega support

Drop the separate Kconfig symbol for Xircom / Entrega and always include
support in the keyspan_pda driver.

Note that all configs that enabled CONFIG_USB_SERIAL_XIRCOM also enable
CONFIG_USB_SERIAL_KEYSPAN_PDA.

Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>