iommupt: Describe @bitnr parameter

iommupt: Describe @bitnr parameter

Sphinx reports kernel-doc warnings when making htmldocs:

WARNING: ./drivers/iommu/generic_pt/pt_common.h:361 function parameter 'bitnr' not described in 'pt_test_sw_bit_acquire'
WARNING: ./drivers/iommu/generic_pt/pt_common.h:371 function parameter 'bitnr' not described in 'pt_set_sw_bit_release'

Describe @bitnr to squash them.

Fixes: bcc64b57b48e ("iommupt: Add basic support for SW bits in the page table")
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>