net: wan: fsl_ucc_hdlc: free tx_skbuff in uhdlc_memclean

net: wan: fsl_ucc_hdlc: free tx_skbuff in uhdlc_memclean

When the device is removed all allocated resources should be freed.
In uhdlc_memclean the netdev transmit queue was already stopped. But at
this point we may have pending skb in the transmit queue which must be
freed. Therefore iterate over the tx_skbuff pointers and free all
pending skb. The issue was discovered by sashiko.
Tested on a ls1043a board running HDLC in bus mode on kernel 6.12.

https: //sashiko.dev/#/patchset/20260429114208.941011-1-holger.brunck%40hitachienergy.com
Fixes: c19b6d246a35 ("drivers/net: support hdlc function for QE-UCC")
Signed-off-by: Holger Brunck <holger.brunck@hitachienergy.com>
Link: https://patch.msgid.link/20260507155332.3452319-1-holger.brunck@hitachienergy.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>