Documentation/mic/mpssd: fix compile error on BE platforms

Change any use of htole* macro in static context to instead manually check
for endianess and use __bswap_constant_* macro instead.

The current glibc definitions of the htole* macros don't allow their use
in a static context; on big endian systems the build fails with:

  HOSTCC  Documentation/mic/mpssd/mpssd.o
In file included from /usr/include/bits/byteswap.h:34:0,
                 from /usr/include/endian.h:60,
                 from /usr/include/bits/waitstatus.h:64,
                 from /usr/include/stdlib.h:42,
                 from /root/linux/Documentation/mic/mpssd/mpssd.c:23:
/root/linux/Documentation/mic/mpssd/mpssd.c:93:10:
error: braced-group within expression allowed only inside a function
   .num = htole16(MIC_VRING_ENTRIES),

...and...

/root/linux/Documentation/mic/mpssd/mpssd.c:119:3:
error: initializer element is not constant
   .host_features = htole32(

I also opened a glibc bug:
https://sourceware.org/bugzilla/show_bug.cgi?id=17679

But this patch is still needed to prevent build failures with the current
glibc htole* macro definitions.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>