usb: Replace snprintf with scnprintf in gether_get_ifname

snprintf returns the actual length of the buffer created; however,
this is not the case if snprintf truncates its parameter.
See https://lwn.net/Articles/69419/

 for a detailed explanation.
The current code correctly handles this case at the expense
of extra code in the return statement.

scnprintf does returns the actual length of the buffer created
making the ?: operator unnecessary in the return
statement.

This change does not alter the functionality of the code.

Signed-off-by: Daniel M German <dmg@turingmachine.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>