Don't use EIMC_OTHER bit because it's read only other than 82598.

Documents say:

  82598:
    All of bit 31(OTHER bit) of EIxx are reserved. In reality, at least
    EIMS_OTHER and EIMC_OTHER exist and the OTHER interrupt doesn't work
    without EIMS_OTHER.

  Other than 82598:
    EIMS_OTHER is read only and EIMC_OTHER doesn't exist. If one of
    bit 29..16 is set, EIMS_OTHER is set to 1 (Note that bit 30(TCP timer
    isn't included)). Even if write bit 31 of EIMC to 1, it's ignored
    (EIMS_OTHER doesn't set).

We introduced new spin mutex in ixgbe.c rev. 1.260, so it's OK to remove
EIMC_OTHER stuff. We already set EIMS_OTHER in if_init(), so keep it for
82598. No functional change other than 82598.

Another solution is to control bit 30..16 directly to mask/unmask interrupt
instead of the mutex.

TODO:
  Some MSI-X interrupt(LSC, module insertion/removal etc.)'s mask/unmask
  code between ixgbe_msix_admin() and ixgbe_handle_admin() may be wrong.
  It'll be fixed later.

(msaitoh)