Set uses_fp for frames with a valid FP register explicitly.

Since CORE_ADDR is unsigned, the saved FP register is always greater than
or equal to zero.  Replace the comparison by explicitly setting uses_fp to
1 for frames with a valid FP register.

gdb/ChangeLog:

	* sh64-tdep.c (sh64_analyze_prologue): Set "uses_fp" when setting
	the MEDIA_FP_REGNUM register.