usb-bsd: fix a file descriptor leak

Fix a file descriptor leak reported by cppcheck:
[/src/qemu/usb-bsd.c:392]: (error) Resource leak: bfd
[/src/qemu/usb-bsd.c:388]: (error) Resource leak: dfd

Rearrange the code to avoid descriptor leaks. Also add braces as
needed.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
