IB/mlx5: Delete locally redefined variable

Fix the following sparse warning:
drivers/infiniband/hw/mlx5/main.c:1061:29: warning: symbol 'pfn' shadows
an earlier one
drivers/infiniband/hw/mlx5/main.c:1030:21: originally declared here

Fixes: d69e3bcf ('IB/mlx5: Mmap the HCA's core clock register to user-space')
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>