ixgbe: correct disable_rx_buff timeout

The current value of the udelay timeout for ixgbe_disable_rx_buff is too
short. This causes the security path to not not be properly disabled during
the section that is meant to have it turned off. The end result causes a race
condition that results in RX issues.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>