hw/misc/tz-mpc: Fix value of BLK_MAX register

In the TZ Memory Protection Controller, the BLK_MAX register is supposed
to return the maximum permitted value of the BLK_IDX register. Our
implementation incorrectly returned max+1 (ie the total number of
valid index values, since BLK_IDX is zero-based).

Correct this off-by-one error. Since we consistently initialize
and use s->blk_max throughout the implementation as the 'size'
of the LUT, just adjust the value we return when the guest reads
the BLK_MAX register, rather than trying to change the semantics
of the s->blk_max internal struct field.

Fixes: https://bugs.launchpad.net/qemu/+bug/1806824


Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181213183249.3468-1-peter.maydell@linaro.org