pcnet32: fix reallocation error

pcnet32_realloc_rx_ring() only worked on the first log2 number of
entries in the receive ring instead of the all the entries.
Replaced "1 << size" with more descriptive variable.
This is my original bug from 2006.  Found while testing another problem.
Tested on 79C972 and 79C976.

Signed-off-by: Don Fry <pcnet32@frontier.com>
Signed-off-by: David S. Miller <davem@davemloft.net>