dfu:tests: Modify dfu_gadget_test.sh to accept USB device vendor:product ID

dfu-util allows filtering on USB device vendor:product ID by using
the -d flag (-d 0451:d022).
Such option is very handy when many DFU devices are connected to a single
host PC. This commit allows testing when above situation emerges.

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Reviewed-by: Simon Glass <sjg@chromium.org>

Tested-by: Lukasz Majewski <l.majewski@majess.pl>
Test HW - AM335x Beagle Bone Black
NOTE: Max size of file to transfer: 2MiB