ethtool: silence warning on bit loss

Sparse was complaining when we went to prototype some code
using ethtool_cmd_speed_set and SPEED_100000, which uses
the upper 16 bits of __u32 speed for the first time.

CHECK
...
.../uapi/linux/ethtool.h:123:28: warning:
  cast truncates bits from constant value (186a0 becomes 86a0)

The warning is actually bogus, as no bits are really lost, but
we can get rid of the sparse warning with this one small change.

Reported-by: Preethi Banala <preethi.banala@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>