ARM: OMAP: omap3evm: fix new sparse warning

Commit e54adb1e ("ARM: OMAP: omap3evm:
cleanup revision bits") adds a new sparse[1] warning:

arch/arm/mach-omap2/board-omap3evm.c:90:4: warning: symbol 'get_omap3_evm_rev' was not declared. Should it be static?

This symbol no longer appears to be used outside this file, so mark it
static and remove the export.

...

1. https://sparse.wiki.kernel.org/index.php/Main_PageSigned-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/board-omap3evm.c

@@ -88,11 +88,10 @@ enum {
 
 static u8 omap3_evm_version;
 
-u8 get_omap3_evm_rev(void)
+static u8 get_omap3_evm_rev(void)
 {
 	return omap3_evm_version;
 }
-EXPORT_SYMBOL(get_omap3_evm_rev);
 
 static void __init omap3_evm_get_revision(void)
 {