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/usbblaster_spi.c b/usbblaster_spi.c
index 886cb57..d6b5084 100644
--- a/usbblaster_spi.c
+++ b/usbblaster_spi.c
@@ -119,11 +119,11 @@
 
 static int send_write(unsigned int writecnt, const unsigned char *writearr)
 {
-	int i;
 	uint8_t buf[BUF_SIZE];
 
 	memset(buf, 0, sizeof(buf));
 	while (writecnt) {
+		unsigned int i;
 		unsigned int n_write = min(writecnt, BUF_SIZE - 1);
 		msg_pspew("writing %d-byte packet\n", n_write);