iommu/amd: Remove redundant assignment to variable npages

The variable npages is being initialized however this is never read and
later it is being reassigned to a new value. The initialization is
redundant and hence can be removed.

Addresses-Coverity: ("Unused Value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>