modules: lvgl: fix build warning for print format

Fix the following building warning:
modules/lvgl/lvgl_shell.c:62:41: warning: format '%u' expects argument
of type 'unsigned int', but argument 3 has type 'size_t' {aka 'long
unsigned int'} [-Wformat=]

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>