USB: gadget: Fix return of -EBUSY

USB: gadget: Fix return of -EBUSY

Currently when driver->match_existing_only is true, the error return is
set to -EBUSY however ret is then set to 0 at the end of the if/else
statement. I believe the ret = 0 statement should be set in the else
part of the if statement and not at the end to ensure -EBUSY is being
returned correctly.

Detected by clang scan:
drivers/usb/gadget/udc/core.c:1558:4: warning: Value stored to 'ret' is
never read [deadcode.DeadStores]

Fixes: fc274c1e9973 ("USB: gadget: Add a new bus for gadgets")
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220504135840.232209-1-colin.i.king@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>