Fix M68HC11_NUM_REGS

Fix M68HC11_NUM_REGS

M68HC11_LAST_HARD_REG is 8, but m68hc11 register number is started from 0,
so there are 9 raw registers, but M68HC11_NUM_REGS is 8 by mistake.

My following unit test can find this issue (GDB is built with asan)

=================================================================
==15555==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000113150 at pc 0x51393f bp 0x7fffcec36f60 sp 0x7fffcec36f58
WRITE of size 2 at 0x602000113150 thread T0
    #0 0x51393e in m68hc11_pseudo_register_read gdb/m68hc11-tdep.c:320
    #1 0xc4b620 in gdbarch_pseudo_register_read(gdbarch*, regcache*, int, unsigned char*) gdb/gdbarch.c:1974
    #2 0xddad88 in regcache::cooked_read(int, unsigned char*) gdb/regcache.c:710
    #3 0xddff2b in cooked_read_test gdb/regcache.c:1850
    #4 0xdf8cfb in selftests::gdbarch_selftest::operator()() const gdb/selftest-arch.c:73

gdb:

2017-11-15  Yao Qi  <yao.qi@linaro.org>

	* m68hc11-tdep.c (M68HC11_NUM_REGS): Change it to
	M68HC11_LAST_HARD_REG + 1.