fm10k: do not clear global mailbox interrupt bits

Partially revert commit 5e93cbad

 ("fm10k: Reset mailbox global
interrupts", 2016-06-07)

The register bits related to this commit are now solely being handled by
the IES API. Recent changes in the IES API will allow an automatic
recovery from improper handling of these bits.

Signed-off-by: Ngai-Mint Kwan <ngai-mint.kwan@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>