RISC-V: Fix register class subset checks for CLASS_MAX_NREGS

Fix the register class subset checks in the determination of the maximum
number of consecutive registers needed to hold a value of a given mode.

The number depends on whether a register is a general-purpose or a
floating-point register, so check whether the register class requested
is a subset (argument 1 to `reg_class_subset_p') rather than superset
(argument 2) of GR_REGS or FP_REGS class respectively.

	gcc/
	* config/riscv/riscv.c (riscv_class_max_nregs): Swap the
	arguments to `reg_class_subset_p'.