drivers: sam_gmac: Fix compilation warning

Line continuation was missing for #error macro.

Fixes  #15096

Signed-off-by: Ravi kumar Veeramally <[email protected]>

Author: rveerama1

drivers/ethernet/eth_sam_gmac.c

@@ -57,9 +57,8 @@ LOG_MODULE_REGISTER(LOG_MODULE_NAME);
 
 #if CONFIG_NET_BUF_DATA_SIZE * (CONFIG_NET_BUF_RX_COUNT - \
 	CONFIG_ETH_SAM_GMAC_BUF_RX_COUNT) < GMAC_FRAME_SIZE_MAX
-#error Remaining free RX data buffers (CONFIG_NET_BUF_RX_COUNT -
-	CONFIG_ETH_SAM_GMAC_BUF_RX_COUNT) * CONFIG_NET_BUF_DATA_SIZE
-	are not large enough to hold a full frame
+#error (CONFIG_NET_BUF_RX_COUNT - CONFIG_ETH_SAM_GMAC_BUF_RX_COUNT) * \
+	CONFIG_NET_BUF_DATA_SIZE are not large enough to hold a full frame
 #endif
 
 #if CONFIG_NET_BUF_DATA_SIZE & 0x3F