Correct pointer type for usb context

[ghost]

Latest master gives the following error on Stretch when make is run. Similar error given on Ubuntu 19.04. PR suggests a change to fix this, but I don't know libusb and the code well enough to know if this is the right fix :) Please review before merging. Thanks. Ray M0DHP

ftdi_usb.c: In function ‘ftdi_usb_init’:
ftdi_usb.c:270:22: error: ‘usb_context’ undeclared (first use in this function)
     libusb_set_debug(usb_context, LIBUSB_LOG_LEVEL_INFO);
                      ^~~~~~~~~~~
ftdi_usb.c:270:22: note: each undeclared identifier is reported only once for each function it appears in
Makefile:28: recipe for target 'ftdi_usb.o' failed
make: *** [ftdi_usb.o] Error 1

[philcrump]

This is correct, thanks for the quick fix Ray!

(Apologies for not fully reading my suggested code)