tipc: fix update of the uninitialized variable err

Variable err is not uninitialized and hence can potentially contain
any garbage value.  This may cause an error when logical or'ing the
return values from the calls to functions crypto_aead_setauthsize or
crypto_aead_setkey.  Fix this by setting err to the return of
crypto_aead_setauthsize rather than or'ing in the return into the
uninitialized variable

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: fc1b6d6d ("tipc: introduce TIPC encryption & authentication")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>