misc: mark write buffer const

misc: mark write buffer const

The write operation in misc_ops already takes a "const void *" buffer,
but misc_write() takes a mutable "void *".  There's no reason for this,
so make misc_write() consistent with the standard write() prototype.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Simon Glass <sjg@chromium.org>