libusbg: Add max and min values to enums

In some cases user of library would like to iterate over
all enum values. Depending on enum values order is not
a good idea so provide suitable min and max values to alow
doing something like:

int i;
for(i = USBG_GADGET_ATTR_MIN; i < USBG_GADGET_ATTR_MAX; ++i)
	do_something_useful(i);

Moreover, ensure that function_names and gadget_attr_names
arrays has allways the same size as number of meaningful
values in related enums.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>