Revert "cmd_net: drop spurious comma in U_BOOT_CMD"

This commit causes build errors like this:

cmd_net.c:301:1: error: macro "U_BOOT_CMD" requires 6 arguments, but only 5 given
cmd_net.c:298: warning: data definition has no type or storage class
cmd_net.c:298: warning: type defaults to 'int' in declaration of 'U_BOOT_CMD'

This reverts commit 8f4cb77e.