USB: usbserial: provide ezusb functions for an external module.

When building an external module, the ezusb_* functions are not defined
if we haven't loaded any built'in module that use them (whiteheat,
keyspan, ...).

This patch allow to build those functions even if we only have selected
the usbserial generic driver.

Signed-off-by: Paul Chavent <paul.chavent@fnac.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>