[intelxl] Fix retrieval of switch configuration via admin queue

[intelxl] Fix retrieval of switch configuration via admin queue

Commit 8f3e648 ("[intelxl] Use one admin queue buffer per admin queue
descriptor") changed the API for intelxl_admin_command() such that the
caller now constructs the command directly within the next available
descriptor ring entry, rather than relying on intelxl_admin_command()
to copy the descriptor to and from the descriptor ring.

This introduced a regression in intelxl_admin_switch(), since the
second and subsequent iterations of the loop will not have constructed
a valid command in the new descriptor ring entry before calling
intelxl_admin_command().

Fix by constructing the command within the loop.

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