tree: Make internal variables static

All these variables are only used in the files they are defined in, so
they can be made static.

Change-Id: I1e55138adef540e9d3a2237aa5b289cb338c0608
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/33747
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/mcp6x_spi.c b/mcp6x_spi.c
index f369277..b53d07b 100644
--- a/mcp6x_spi.c
+++ b/mcp6x_spi.c
@@ -36,7 +36,7 @@
 #define MCP6X_SPI_REQUEST	0
 #define MCP6X_SPI_GRANT		8
 
-void *mcp6x_spibar = NULL;
+static void *mcp6x_spibar = NULL;
 
 /* Cached value of last GPIO state. */
 static uint8_t mcp_gpiostate;