[intel] Remove hardcoded offsets for descriptor ring registers

[intel] Remove hardcoded offsets for descriptor ring registers

The Intel 10 Gigabit NICs use the same simplified (aka "legacy")
descriptor format and the same layout for descriptor register blocks
as the Intel 1 Gigabit NICs.  The offsets of the descriptor register
blocks are not the same.

Simplify reuse of the existing code by removing all hardcoded offsets
for registers within descriptor register blocks, and ensuring that all
offsets are calculated using the descriptor register block base
address provided via intel_init_ring().

Signed-off-by: Michael Brown <mcb30@ipxe.org>