ixgbe: add braces around else block

This commit fixes a checkpatch.pl warning for style, by adding braces
around the else block, since the if block requires braces.

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>