net: ethernet: rmnet: Restructure if checks to avoid uninitialized warning

Clang warns that proto in rmnet_map_v5_checksum_uplink_packet() might be
used uninitialized:

drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:283:14: warning:
variable 'proto' is used uninitialized whenever 'if' condition is false
[-Wsometimes-uninitialized]
                } else if (skb->protocol == htons(ETH_P_IPV6)) {
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:295:36: note:
uninitialized use occurs here
                check = rmnet_map_get_csum_field(proto, trans);
                                                 ^~~~~
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:283:10: note:
remove the 'if' if its condition is always true
                } else if (skb->protocol == htons(ETH_P_IPV6)) {
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:270:11: note:
initialize the variable 'proto' to silence this warning
                u8 proto;
                        ^
                         = '\0'
1 warning generated.

This is technically a false positive because there is an if statement
above this one that checks skb->protocol for not being either
ETH_P_IP{,V6}. However, it is more obvious to sink that into the if
statement as an else branch, which makes the code clearer and fixes the
warning.

At the same time, move the "IS_ENABLED(CONFIG_IPV6)" into the else if
condition so that the else branch of the preprocessor conditional can
be shared, since there is no build failure with CONFIG_IPV6 disabled.

Fixes: b6e5d27e

 ("net: ethernet: rmnet: Add support for MAPv5 egress packets")
Link: https://github.com/ClangBuiltLinux/linux/issues/1390
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>