if_smsc: fix build on armv6 & armv7

compile error was:
/usr/src/sys/dev/usb/net/if_smsc.c:1597:40: error: format specifies type 'unsigned long' but the argument has type 'ssize_t' (aka 'int') [-Werror,-Wformat]

"failed alloc for bootargs (%lu)", len);
                            ~~~    ^~~
                            %zd

PR: 274092
Approved by: karels
MFC after: 1 month