cxgb4/cxgb4vf: read the correct bits of PL Who Am I register

Read the correct bits of PL Who Am I for the Source PF field which has
changed in T6

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>