USB: Add helper macro for usb_driver boilerplate

This patch introduces the module_usb_driver macro which is a convenience
macro for USB driver modules similar to module_platform_driver. It is
intended to be used by drivers which init/exit section does nothing but
register/unregister the USB driver. By using this macro it is possible
to eliminate a few lines of boilerplate code per USB driver.

Based on work done by Lars-Peter Clausen <lars@metafoo.de> for other
busses (i2c and spi).

Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>