net: stmmac: Avoid sometimes uninitialized Clang warnings

When building with -Wsometimes-uninitialized, Clang warns:

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:495:3: warning: variable 'ns' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:495:3: warning: variable 'ns' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]
drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:532:3: warning: variable 'ns' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:532:3: warning: variable 'ns' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]
drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:741:3: warning: variable 'sec_inc' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:741:3: warning: variable 'sec_inc' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]

Clang is concerned with the use of stmmac_do_void_callback (which
stmmac_get_timestamp and stmmac_config_sub_second_increment wrap),
as it may fail to initialize these values if the if condition was ever
false (meaning the callbacks don't exist). It's not wrong because the
callbacks (get_timestamp and config_sub_second_increment respectively)
are the ones that initialize the variables. While it's unlikely that the
callbacks are ever going to disappear and make that condition false, we
can easily avoid this warning by zero initialize the variables.

Link: https://github.com/ClangBuiltLinux/linux/issues/384


Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>