Revert change to cyw43_hal_generate_laa_mac (#1291)

This was changed to use the same algorithm as MP.

This function is used to set the wifi mac if OTP is unset (which
shouldn't ever be the case). But if it does occur, changing the mac
might cause issues, so lets play safe and revert the change.
diff --git a/src/rp2_common/pico_cyw43_driver/cyw43_driver.c b/src/rp2_common/pico_cyw43_driver/cyw43_driver.c
index e63a9fa..6a3d01c 100644
--- a/src/rp2_common/pico_cyw43_driver/cyw43_driver.c
+++ b/src/rp2_common/pico_cyw43_driver/cyw43_driver.c
@@ -129,16 +129,13 @@
 }
 
 // Generate a mac address if one is not set in otp
-void __attribute__((weak)) cyw43_hal_generate_laa_mac(int idx, uint8_t buf[6]) {
+void __attribute__((weak)) cyw43_hal_generate_laa_mac(__unused int idx, uint8_t buf[6]) {
     CYW43_DEBUG("Warning. No mac in otp. Generating mac from board id\n");
-    pico_unique_board_id_t pid;
-    pico_get_unique_board_id(&pid);
-    buf[0] = 0x02; // LAA range
-    buf[1] = (pid.id[7] << 4) | (pid.id[6] & 0xf);
-    buf[2] = (pid.id[5] << 4) | (pid.id[4] & 0xf);
-    buf[3] = (pid.id[3] << 4) | (pid.id[2] & 0xf);
-    buf[4] = pid.id[1];
-    buf[5] = (pid.id[0] << 2) | idx;
+    pico_unique_board_id_t board_id;
+    pico_get_unique_board_id(&board_id);
+    memcpy(buf, &board_id.id[2], 6);
+    buf[0] &= (uint8_t)~0x1; // unicast
+    buf[0] |= 0x2; // locally administered
 }
 
 // Return mac address