module/raid: convert spaces to tabs in one .h struct

Adding a new element to this struct in an upcoming patch and want
it to look pretty and be consistent with everything else.

Signed-off-by: paul luse <paul.e.luse@intel.com>
Change-Id: I514244ea9441081ba83bd7425101a2bd3ba532c1
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/467885


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>