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: 2b0569b3 ("NTB: Add MSI interrupt support to ntb_transport")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>