x86, olpc: fix endian bug in openfirmware workaround

Boardrev is always treated as a u32 everywhere else, no reason to
byteswap the 0xc2 value.  The only use is to print out if it is
a prerelease board, the test being:

(olpc_platform_info.boardrev & 0xf) < 8

Which is currently always true as be32_to_cpu(0xc2) & 0xf = 0
but I doubt that was the intention here.  The consequences of the bug
are pretty minor though (incorrect boardrev displayed in dmesg when
ofw support not configured)

Also annotate the temporary used to read the boardrev in the ofw
case.

The confusion was noticed by Sparse:

  arch/x86/kernel/olpc.c:206:32: warning: cast to restricted __be32

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>