target/arm: Fix nregs computation in do_{ld,st}_zpa

target/arm: Fix nregs computation in do_{ld,st}_zpa

The field is encoded as [0-3], which is convenient for
indexing our array of function pointers, but the true
value is [1-4].  Adjust before calling do_mem_zpa.

Add an assert, and move the comment re passing ZT to
the helper back next to the relevant code.

Cc: qemu-stable@nongnu.org
Fixes: 206adacfb8d ("target/arm: Add mte helpers for sve scalar + int loads")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Gustavo Romero <gustavo.romero@linaro.org>
Message-id: 20240207025210.8837-3-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>