gdbtypes: return value from get_unsigned_type_max

gdbtypes: return value from get_unsigned_type_max

Changes the signature of get_unsigned_type_max to return the computed
value rather than returning void and writing the value into a pointer
passed by the caller.

gdb/ChangeLog:

2021-07-30  George Barrett  <bob@bob131.so>

	* gdbtypes.h (get_unsigned_type_max): Change signature to
	return the result instead of accepting a pointer argument in
	which to store the result.
	* gdbtypes.c (get_unsigned_type_max): Likewise.
	* guile/scm-math.c (vlscm_convert_typed_number): Update caller
	of get_unsigned_type_max.
	(vlscm_integer_fits_p): Likewise.

Change-Id: Ibb1bf0c0fa181fac7853147dfde082a7d1ae2323