tools: usbip: detach: avoid calling strlen() at each iteration

Instead of calling strlen on every iteration of the for loop, just call it
once and cache the result in a temporary local variable which will be used
in the for loop instead.

Signed-off-by: Eric Curtin <ericcurtin17@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>