usb: fix ipw.c build error, select USB_SERIAL_WWAN

USB_SERIAL_IPW needs to select USB_SERIAL_WWAN to fix build errors:

ipw.c:(.text+0x333841): undefined reference to `usb_wwan_close'
ipw.c:(.text+0x3339e6): undefined reference to `usb_wwan_open'
ipw.c:(.text+0x333bb9): undefined reference to `usb_wwan_release'
drivers/built-in.o:(.data+0x29dac): undefined reference to `usb_wwan_startup'
drivers/built-in.o:(.data+0x29db4): undefined reference to `usb_wwan_disconnect'
drivers/built-in.o:(.data+0x29dd4): undefined reference to `usb_wwan_write'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>