Consolidate ARRAY_SIZE macro; remove one unneeded global var (walter harms <wharms@bfs.de>)

diff --git a/libbb/speed_table.c b/libbb/speed_table.c
index 6137b77..0f0b33d 100644
--- a/libbb/speed_table.c
+++ b/libbb/speed_table.c
@@ -54,7 +54,7 @@
 #endif
 };
 
-enum { NUM_SPEEDS = (sizeof(speeds) / sizeof(struct speed_map)) };
+enum { NUM_SPEEDS = ARRAY_SIZE(speeds) };
 
 unsigned int tty_baud_to_value(speed_t speed)
 {