wctc4xxp: Ensure the descriptors are zeroed out on start.

If system conditions prevent the card from loading the firmware without
touching all the descriptors in the ring, it's possible for
wctc4xxp_cleanup_descriptor_ring to attempt cleanup of a command that was never
submitted resulting in the following BUG.

  wctc4xxp 0000:02:0a.0: Failed to load firmware.
  BUG: unable to handle kernel paging request at c109556c
  IP: [<f9faa83e>] wctc4xxp_cleanup_descriptor_ring+0x8c/0x148 [wctc4xxp]

Reported-by: Xavier Carcelle <xcarcelle@avencall.com>
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
(cherry picked from commit 040ba6f43a3cd519c8cf4c996fb6d6ed117e0a1a)