cdc-wdm: Don't clear WDM_READ unless entire read buffer is emptied

commit b7a205545345578712611106b371538992e142ff upstream.

The WDM_READ flag is cleared later iff desc->length is reduced to 0.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Bjørn Mork <bjorn@mork.no>
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>