[autoboot] Fix shell banner timeout

Commit 5e1fa5c ("[parseopt] Add parse_timeout()") introduced a
regression causing the shell banner timeout value (calculated in
milliseconds) to be treated as a timer tick count, resulting in a
timeout of approximately two minutes rather than the intended two
seconds.

Reported-by: Christian Hesse <list@eworm.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/usr/autoboot.c

 #include <ipxe/shell.h>
 #include <ipxe/features.h>
 #include <ipxe/image.h>
+#include <ipxe/timer.h>
 #include <usr/ifmgmt.h>
 #include <usr/route.h>
 #include <usr/dhcpmgmt.h>
 	/* Prompt user */
 	printf ( "\n" );
 	return ( prompt ( "Press Ctrl-B for the iPXE command line...",
-			  ( BANNER_TIMEOUT * 100 ), CTRL_B ) == 0 );
+			  ( ( BANNER_TIMEOUT * TICKS_PER_SEC ) / 10 ),
+			  CTRL_B ) == 0 );
 }
 
 /**