usb: gadget: u_ether: fix debug format string

We get a harmless warning on 64-bit architectures about a format string:

drivers/usb/gadget/function/u_ether.c: In function 'rx_submit':
include/linux/kern_levels.h:4:18: warning: format '%d' expects argument of type 'int', but argument 4 has type 'size_t {aka long unsigned int}' [-Wformat=]

This changes the code to use the correct %zd for size_t.

Link: https://kernelci.org/build/id/5910609459b514c7cbb12d50/logs/


Fixes: c5ecbca4 ("usb: Add support for rndis uplink aggregation")
Cc: Xerox Lin <xerox_lin@htc.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>