USB: gadget: u_f: Unbreak offset calculation in VLAs

USB: gadget: u_f: Unbreak offset calculation in VLAs

Inadvertently the commit b1cd1b65afba ("USB: gadget: u_f: add overflow checks
to VLA macros") makes VLA macros to always return 0 due to different scope of
two variables of the same name. Obviously we need to have only one.

Fixes: b1cd1b65afba ("USB: gadget: u_f: add overflow checks to VLA macros")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Brooke Basile <brookebasile@gmail.com>
Cc: stable <stable@kernel.org>
Link: https://lore.kernel.org/r/20200826192119.56450-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>