net: dwc_eth_qos: Fix compilation warning in eqos_free_pkt()

Fix compilation warning:

../arch/arm/include/asm/io.h: In function 'eqos_free_pkt':
../arch/arm/include/asm/io.h:103:32: warning: 'rx_desc' may be used uninitialized [-Wmaybe-uninitialized]
  103 | #define writel(v,c)     ({ u32 __v = v; __iowmb(); __arch_putl(__v,c); __v; })
      |                                ^~~
../drivers/net/dwc_eth_qos.c:1220:27: note: 'rx_desc' was declared here
 1220 |         struct eqos_desc *rx_desc;
      |                           ^~~~~~~

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>