ether82563, ether82598, etherx550: round rbsz to multiple of 1K

ether82563, ether82598, etherx550: round rbsz to multiple of 1K

the max packet size is configured in 1K increments on these chips,
which can result in the card receiving a 10K packet but the
driver having only allocated 9.5K of buffer. this actually caued
pool corruption with i210, i217, i218, i219, i350.

for 82598 and x550, we explicitely round rbsz to avoid similar bugs
in the future, even tho the Rbsz constant was already a multiple of
1K and is not affected by the bug.