[obexd] Retry opening ttyGS0 if it's not there. Fixes JB#12429.

Opening of ttyGS0 triggered by usb mode change fails sometimes
because the file does not yet exist. Work around that by trying
a few times.
Merge pull request #9 from hmallat/usb-retry-tty

[obexd] Retry opening ttyGS0 if it's not there. Fixes JB#12429.