[romprefix] Update ROM checksum even if PMM allocation fails

There are code paths other than PMM allocation that can result in our
changing the ROM checksum.  For example, we attempt to update our
product string to incorporate the PCI bus:dev.fn number.  In a system
that does not support PMM, we could therefore end up with an incorrect
checksum.

Fix by attempting to update the checksum unconditionally.

src/arch/i386/prefix/romprefix.S

 	shll	$9, %ecx
 	addr32 rep movsb	/* PMM presence implies flat real mode */
 	movl	%edi, decompress_to
-	/* Shrink ROM and update checksum */
+	/* Shrink ROM */
+	movb	$_prefix_memsz_sect, romheader_size
+pmm_fail:
+	/* Restore upper register halves */
+	popal
+no_pmm:
+
+	/* Update checksum */
 	xorw	%bx, %bx
 	xorw	%si, %si
-	movw	$_prefix_memsz_sect, %cx
-	movb	%cl, romheader_size
+	movzbw	romheader_size, %cx
 	shlw	$9, %cx
 1:	lodsb
 	addb	%al, %bl
 	loop	1b
 	subb	%bl, checksum
-pmm_fail:
-	/* Restore upper register halves */
-	popal
-no_pmm:
 
 	/* Copy self to option ROM space.  Required for PCI3.0, which
 	 * loads us to a temporary location in low memory.  Will be a