ixgbe: fix bitfield assignation with clang

ixgbe: fix bitfield assignation with clang

Commit 1224decaa44 ("support TCP segmentation offload")
changed the way the bitfields are assigned in ixgbe, example:

  tx_offload_mask.l2_len = ~0;

This result in a compilation error with clang:

  error: implicit truncation from 'int' to bitfield
    changes value from -1 to 127 [-Werror,-Wbitfield-constant-conversion]

Replacing the '=' with a '|=' fixes the issue.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>