ntb: use 64-bit arithmetic for the MSI doorbell mask

msi_db_mask is of type 'u64', still the standard 'int' arithmetic is
performed to compute its value.

While most of the ntb_hw drivers actually don't utilize the higher 32
bits of the doorbell mask now, this may be the case for Switchtec - see
switchtec_ntb_init_db().

Found by Linux Verification Center (linuxtesting.org) with SVACE static
analysis tool.

Fixes: 2b0569b3b7e6 ("NTB: Add MSI interrupt support to ntb_transport")
Cc: [e-mail]
Signed-off-by: Fedor Pchelkin <[e-mail]>
Reviewed-by: Dave Jiang <[e-mail]>
Signed-off-by: Jon Mason <[e-mail]>

drivers/ntb/ntb_transport.c

···
 	qp_count = ilog2(qp_bitmap);
 	if (nt->use_msi) {
 		qp_count -= 1;
-		nt->msi_db_mask = 1 << qp_count;
+		nt->msi_db_mask = BIT_ULL(qp_count);
 		ntb_db_clear_mask(ndev, nt->msi_db_mask);
 	}