drivers: net: smc91x: Fix set but unused W=1 warning

drivers/net/ethernet/smsc/smc91x.c:706:51: warning: variable ‘pkt_len’ set but not used [-Wunused-but-set-variable]
  706 |  unsigned int saved_packet, packet_no, tx_status, pkt_len;

The read of the packet length in the descriptor probably needs to be
kept in order to keep the hardware happy. So tell the compiler we
don't expect to use the value by using the __always_unused attribute.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>