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

[Eric Curtin <ericcurtin17@gmail.com>]

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>

diff --git a/tools/usb/usbip/src/usbip_detach.c b/tools/usb/usbip/src/usbip_detach.c
index 05c6d15..9db9d21 100644
--- a/tools/usb/usbip/src/usbip_detach.c
+++ b/tools/usb/usbip/src/usbip_detach.c
@@ -47,7 +47,9 @@ static int detach_port(char *port)
 	uint8_t portnum;
 	char path[PATH_MAX+1];
 
-	for (unsigned int i = 0; i < strlen(port); i++)
+	unsigned int port_len = strlen(port);
+
+	for (unsigned int i = 0; i < port_len; i++)
 		if (!isdigit(port[i])) {
 			err("invalid port %s", port);
 			return -1;