i2c-pmcmsp: Fix endianness misannotation

tmp is used as host-endian and is loaded from a be64, fix the cast and the
endian accessor used.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>