udev-builtin-net_id: fix construction of USB specifier-based names

udev-builtin-net_id: fix construction of USB specifier-based names

Commit 0bac1ed242 ("tree-wide: Fix constness issues with newer glibc")
split a temporary var in get_usb_specifier to const and non-const
versions, but missed converting a couple of uses. This means we get
names of with components of:

 port: uN
 config: cC.I
 interface: iC.I

instead of:

 port: uN
 config: cC
 interface: iI

This results in net names like enu1c1.0i1.0, as we also no longer hit
the config==1 and interf==0 elision cases.

Change the config portion handling to start from the correct position,
and the earlier check for NULL.