ibmvnic: Use/rename local vars in init_rx_pools

To make the code more readable, use/rename some local variables.
Basically we have a set of pools, num_pools. Each pool has a set of
buffers, pool_size and each buffer is of size buff_size.

pool_size is a bit ambiguous (whether size in bytes or buffers). Add
a comment in the header file to make it explicit.

Reviewed-by: Rick Lindsley <ricklind@linux.vnet.ibm.com>
Reviewed-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>