usb: early: xhci-dbc: Optimize early_xdbc_write()

There is no point in zeroing 'buf'.
It would be cleared only once, and if the 'while' loop is executed several
times, all but the first run would have a 'dirty' buffer.
Moreover, the size of the chunk is computed in the loop and this size is
passed to xdbc_bulk_write().

So remove this useless memset().

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/687bbcd940c59fbddd0e3a8b578fd3422962e50f.1675016180.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>