net: ravb: Fix -Wmaybe-uninitialized warning

net: ravb: Fix -Wmaybe-uninitialized warning

Fix a -Wmaybe-uninitialized warning by initializing the variable to
NULL. The warning is bogus and should not happen, but fixing it allows
running the check on the driver to catch potential future problems.

  $ make CFLAGS_ravb_main.o=-Wmaybe-uninitialized

  In function 'ravb_rx_csum_gbeth',
      inlined from 'ravb_rx_gbeth' at .../linux/drivers/net/ethernet/renesas/ravb_main.c:923:6:
  .../linux/drivers/net/ethernet/renesas/ravb_main.c:765:25: error: 'skb' may be used uninitialized [-Werror=maybe-uninitialized]
    765 |         if (unlikely(skb->len < csum_len))
        |                      ~~~^~~~~
  .../linux/include/linux/compiler.h:77:45: note: in definition of macro 'unlikely'
     77 | # define unlikely(x)    __builtin_expect(!!(x), 0)
        |                                             ^
  .../linux/drivers/net/ethernet/renesas/ravb_main.c: In function 'ravb_rx_gbeth':
  .../linux/drivers/net/ethernet/renesas/ravb_main.c:806:25: note: 'skb' was declared here
    806 |         struct sk_buff *skb;
        |                         ^~~
  cc1: all warnings being treated as errors

Warning was found when cross compiling using aarch64-linux-gnu-gcc (GCC)
15.1.0.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://patch.msgid.link/20250913133229.2087822-1-niklas.soderlund+renesas@ragnatech.se
Signed-off-by: Jakub Kicinski <kuba@kernel.org>