vendorcode/cavium: Use unsigned integers in struct bitfields

Bitfields with signed integers are not valid C code. This fixes
compilation with clang v16.0.6.

Change-Id: I0b2add2f1078a88347fea7dc65d422d0e5a210a1
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80638
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/vendorcode/cavium/include/bdk/libbdk-hal/if/bdk-if.h

@@ -113,8 +113,8 @@ typedef union
         uint64_t    reserved2   : 32;
         unsigned    speed       : 24;
         unsigned    lanes       : 6;
-        int         full_duplex : 1;
+        unsigned    full_duplex : 1;
-        int         up          : 1;
+        unsigned    up          : 1;
     } s;
 } bdk_if_link_t;