Restrict m68k_convert_register_p

Restrict m68k_convert_register_p

We need to convert register if the type is float.  Suppose we get a value
from float point register, but its type is integer, we don't have to convert.
This case may not exist in real code, but exist in my unit test case.

warning: Cannot convert floating-point register value to non-floating-point type.
Self test failed: arch m68k: self-test failed at gdb/git/gdb/findvar.c:1072

              ok = gdbarch_register_to_value (gdbarch, frame, regnum, type,
                                              buf.data (), &optim, &unavail);

1072:         SELF_CHECK (ok);

gdb:

2017-05-24  Yao Qi  <yao.qi@linaro.org>

	* m68k-tdep.c (m68k_convert_register_p): Check type's code is
	TYPE_CODE_FLT or not.