usb: move connection string parsing out of USB enumeration/search

usb: move connection string parsing out of USB enumeration/search

The sr_usb_find() routine inspects an input specification in textual
format, as well as scans for USB devices that match the caller specified
address. Factor out the text parsing including range checks for address
numbers. This part of the source code also does not depend on the libusb
library's availability.

Adjust the sr_usb_find() routine's data types for result variables. This
commit changes user visible diagnostics messages, since text parsing and
USB device lookup happen in distinct locations and don't share knowledge.