ixgbe: consolidate reporting of MSIX vectors into a single function

This patch modifies ixgbe_get_pcie_msix_count_generic() to support
all current HW and removes the 82598 specific function.

- change the type of ixgbe_get_pcie_msix_count_generic() to u16
- include a check to make sure the maximum allowed number of vectors
is not exceeded.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>