RISC-V: Fix riscv_vector_chunks configuration according to TARGET_MIN_VLEN

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_convert_vector_bits): Change
	configuration according to TARGET_MIN_VLEN.
	* config/riscv/riscv.h (UNITS_PER_FP_REG): Fix comment.