vsprintf: use TOLOWER whenever possible

It decreases code size as well:
 text    data     bss     dec     hex filename
15758       0       8   15766    3d96 vsprintf.o (ex lib/lib.a-BEFORE)
15726       0       8   15734    3d76 vsprintf.o (ex lib/lib.a-TOLOWER)

Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>