lib: Add error detection for misa_extension

Add assertions for misa_extension to prevent incoming illegal
characters.

Signed-off-by: Xiang Wang <merle@hardenedlinux.org>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

lib/sbi/riscv_asm.c

@@ -12,7 +12,8 @@
 #include <sbi/sbi_error.h>
 #include <sbi/sbi_platform.h>
 
-int misa_extension(char ext)
+/* determine CPU extension, return non-zero support */
+int misa_extension_imp(char ext)
 {
 	unsigned long misa = csr_read(CSR_MISA);