Fix -Wsign-compare trouble

Mostly by changing to `unsigned` types where applicable, sometimes
`signed` types, and casting as a last resort.

Change-Id: I08895543ffb7a48058bcf91ef6500ca113f2d305
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/30409
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jacob Garber <jgarber1@ualberta.ca>
diff --git a/serprog.c b/serprog.c
index b6c85c9..37a9db4 100644
--- a/serprog.c
+++ b/serprog.c
@@ -71,8 +71,8 @@
 static uint32_t sp_write_n_bytes = 0;
 
 /* sp_streamed_* used for flow control checking */
-static int sp_streamed_transmit_ops = 0;
-static int sp_streamed_transmit_bytes = 0;
+static unsigned int sp_streamed_transmit_ops = 0;
+static unsigned int sp_streamed_transmit_bytes = 0;
 
 /* sp_opbuf_usage used for counting the amount of
 	on-device operation buffer used */