mve: Fix encoding for vcvt[bt] single-half float conversion instructions

The encoding was previously not taking into account that the Quad vector
registers were being encoded using their Q-register numbers rather than their
D-register equivalent (multiply by 2).

gas/

	* config/tc-arm.c (do_neon_cvttb_1): Use Q-register vector number
	rather than their D-register equivalent.

gas/testsuite/

	* gas/arm/mve-vcvt-3.d: Correct expected values in test.