ixgbe: refactor: use DECLARE_BITMAP for ring state field

ixgbe: refactor: use DECLARE_BITMAP for ring state field

Convert the ring state field from 'unsigned long' to a proper bitmap
using DECLARE_BITMAP macro, aligning with the implementation pattern
already used in the i40e driver.

This change:
- Adds __IXGBE_RING_STATE_NBITS as the bitmap size sentinel to enum
  ixgbe_ring_state_t (consistent with i40e's __I40E_RING_STATE_NBITS)
- Changes 'unsigned long state' to 'DECLARE_BITMAP(state,
  __IXGBE_RING_STATE_NBITS)' in struct ixgbe_ring
- Removes the address-of operator (&) when passing ring->state to bit
  manipulation functions, as bitmap arrays naturally decay to pointers

The change maintains functional equivalence while using the
more appropriate kernel bitmap API, consistent with other Intel Ethernet
drivers.

Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Marcin Szycik <marcin.szycik@linux.intel.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Tested-by: Rinitha S <sx.rinitha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>