cmd: disk: Fix unused variable warning

cmd: disk: Fix unused variable warning

If serial support is not compiled into U-Boot, which may be the case
for some SPL builds, the following warning will be generated in disk.c:

cmd/disk.c: In function 'common_diskboot':
cmd/disk.c:16:6: warning: variable 'dev' set but not used [-Wunused-but-set-variable]
  int dev, part;
      ^
The warning is a result of printf() calls being optimized away, and
thus the whole dev variable becomes indeed unused. Mark the variable
as __maybe_unused .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>