ixgbe: fix issues setting rx-usecs with legacy interrupts

Currently setting rx-usecs when the interface is in legacy interrupt
mode it is not immediate.  We were only setting EITR for each MSIx
vector and since this count would be zero for legacy mode it wasn't
set until after a reset.  This patch corrects that by checking what
mode we are in and then setting EITR accordingly.

Signed-off-by: Don Skidmore <donald.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>