Avoid undefined behaviour in m68hc11 md_begin

Avoid undefined behaviour in m68hc11 md_begin

Given p = A where p is a pointer to some type and A is an array of
that type, then the expression p - 1 + 1 evokes undefined behaviour
according to the C standard.

gcc-13 -fsanitize=address,undefined complains about this, but not
where the undefined behaviour actually occurs at tc-m68hc11.c:646.
Instead you get an error: "tc-m68hc11.c:708:20: runtime error: store
to address 0x62600000016c with insufficient space for an object of
type 'int'".  Which is a lie.  There most definitely is space there.
Oh well, diagnostics are sometimes hard to get right.  The UB is easy
to avoid.

	PR 30279
	* config/tc-m68hc11.c (md_begin): Avoid undefined pointer
	decrement.  Remove unnecessary cast.