usb: gadget: fastboot: terminate commands with NULL

Without NULL termination, various commands will read past the
end of input. In particular, this was noticed with error()
calls in cb_getvar and simple_strtoul() in cb_download.

Since the download callback happens elsewhere, the 4k buffer
should always be sufficient to handle command arguments.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>