omap2/3/4: serial: kill dev_attr_sleep_timeout sparse warn

Remove the following sparse warnings by declaring attr as static:
arch/arm/mach-omap2/serial.c:627:1: warning: symbol 'dev_attr_sleep_timeout'
was not declared. Should it be static?

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>