ch_ktls: fix build warning for ipv4-only config

When CONFIG_IPV6 is disabled, clang complains that a variable
is uninitialized for non-IPv4 data:

drivers/net/ethernet/chelsio/inline_crypto/ch_ktls/chcr_ktls.c:1046:6: error: variable 'cntrl1' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
        if (tx_info->ip_family == AF_INET) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/chelsio/inline_crypto/ch_ktls/chcr_ktls.c:1059:2: note: uninitialized use occurs here
        cntrl1 |= T6_TXPKT_ETHHDR_LEN_V(maclen - ETH_HLEN) |
        ^~~~~~

Replace the preprocessor conditional with the corresponding C version,
and make the ipv4 case unconditional in this configuration to improve
readability and avoid the warning.

Fixes: 86716b51

 ("ch_ktls: Update cheksum information")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20201203222641.964234-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>