bnxt_en: Optimize ring alloc and ring free functions.

Remove the unnecessary "if" statement before the "for" statement:

if (x) {
        for (i = 0; i < x; i++)
...
}

Also, change the ring free function to return void as it only returns 0.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>