net: ks8851: Make ks8851_read_selftest() return void

clang points out that ret in ks8851_read_selftest() is set but unused:

drivers/net/ethernet/micrel/ks8851_common.c:1028:6: warning: variable
'ret' set but not used [-Wunused-but-set-variable]
        int ret = 0;
            ^
1 warning generated.

The return code of this function has never been checked so just remove
ret and make the function return void.

Fixes: 3ba81f3e

 ("net: Micrel KS8851 SPI network driver")
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>