tfm: use unsigned int for fp_digit on !FP_64BIT

There is a bugzilla entry at clamav [0] reports a problem on s390x
  "mul/fp_mul_comba_20.c:91: warning: right shift count >= width of type"
because fp_digit is 64bit and according to the comment it should be
smaller.
s390x is 64bit platform i.e. long and long long are both 64bit. The
same is true for ppc64, arm64 or sparc64 (RIP alpha).

The headerfile seems only to define FP_64BIT on x86_64.
The current code seems to define SIZEOF_FP_DIGIT either to 8 (64bit) or
to 4 in the else case and 4 looks like int and not long. Therefore the
change here.

If this change makes no sense and you would prefer to use something like
    #if __SIZEOF_LONG__ == 8
       #define FP_64BIT
    #endif

to autodetect a 64bit platform then we could try do this instead. I
haven't tested this patch nor verified that the problem still occures.

If you have no obvious solution and would like to have a confirmation
that this patch solves a problem I could try to get my hands on
something not x86 with 64bit in order to verify.

[0] https://bugzilla.clamav.net/show_bug.cgi?id=9017

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

src/headers/tfm.h

@@ -265,7 +265,7 @@
    #endif /* defined(_MSC_VER) ... */
 #endif /* CRYPT */
 
-   typedef unsigned long      fp_digit;
+   typedef unsigned int       fp_digit;
 #define SIZEOF_FP_DIGIT 4
    typedef ulong64            fp_word;
 #endif /* FP_64BIT */