x86/events/amd/iommu: Remove redundant assignment to variable shift

Variable shift is being initialized with a value that is never read, it
is being re-assigned later inside a loop. The assignment is redundant
and can be removed.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211207185001.1412413-1-colin.i.king@gmail.com