mei: bus: elminate variable length arrays

Though VLA are supported by CC99 there are many cavities
and should be avoided.

'const size_t len = sizeof()' that we used may not be set
at the compile time hence generating VLA code.

This fixes also sparse warning
warning: Variable length array is used type.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>