wil6210: fix RX checksum report to network stack

Currently the driver sets CHECKSUM_UNNECESSARY only in case the HW
doesn't report checksum error.
As ip_summed value is not initialized it is not clear what
the driver will report to the network stack in case of HW checksum
error or in case HW doesn't calculate checksum.
Initialize ip_summed to CHECKSUM_NONE to guarantee checksum
calculation by the network stack in the above cases.

Change-Id: I1c0d3301201e49992adafbc02e37cca822f8bae5
Signed-off-by: Gidon Studinski <gidons@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>