crypto: make mod_exp_sw() static

Function mod_exp_sw() is only used via the operators of the uclass.
It is not defined in any include.

Make mod_exp_sw() static.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/drivers/crypto/rsa_mod_exp/mod_exp_sw.c b/drivers/crypto/rsa_mod_exp/mod_exp_sw.c
index 5a098f8..46b9f18 100644
--- a/drivers/crypto/rsa_mod_exp/mod_exp_sw.c
+++ b/drivers/crypto/rsa_mod_exp/mod_exp_sw.c
@@ -9,8 +9,8 @@
 #include <dm.h>
 #include <u-boot/rsa-mod-exp.h>
 
-int mod_exp_sw(struct udevice *dev, const uint8_t *sig, uint32_t sig_len,
-		struct key_prop *prop, uint8_t *out)
+static int mod_exp_sw(struct udevice *dev, const uint8_t *sig, uint32_t sig_len,
+		      struct key_prop *prop, uint8_t *out)
 {
 	int ret = 0;