[crypto] Add parentheses around len argument in blocksize assert

This fixes an issue where passing a length as a compound expression
(e.g. using `hdrlen + datalen') would trigger compiler warnings and
potentially precedence-related errors.

Signed-off-by: Marty Connor <mdc@etherboot.org>

src/include/gpxe/crypto.h

 	cipher->encrypt ( ctx, src, dst, len );
 }
 #define cipher_encrypt( cipher, ctx, src, dst, len ) do {		\
-	assert ( ( len & ( (cipher)->blocksize - 1 ) ) == 0 );		\
+	assert ( ( (len) & ( (cipher)->blocksize - 1 ) ) == 0 );	\
 	cipher_encrypt ( (cipher), (ctx), (src), (dst), (len) );	\
 	} while ( 0 )
 
 	cipher->decrypt ( ctx, src, dst, len );
 }
 #define cipher_decrypt( cipher, ctx, src, dst, len ) do {		\
-	assert ( ( len & ( (cipher)->blocksize - 1 ) ) == 0 );		\
+	assert ( ( (len) & ( (cipher)->blocksize - 1 ) ) == 0 );	\
 	cipher_decrypt ( (cipher), (ctx), (src), (dst), (len) );	\
 	} while ( 0 )