s2io: rx_ring_sz bounds checking

modparm rx_ring_sz can be set to be greater than the maximum allowable
number of blocks.  This results in an array overrun when probing the
driver, and causes memory corruption.

Also, the MAX_RX_DESC_1 multiply the max number of rings by max number
of blocker per ring by 127, but the driver does the same calculation
with 127+1.  This results in the possibility of the value being set
being larger than the maximum allowable value.

Finally, clean-up the s2io_ethtool_gringparam code to be more
intuitive.

Signed-off-by: Jon Mason <jon.mason@exar.com>
Signed-off-by: David S. Miller <davem@davemloft.net>