x86: early_printk.c trivial sparse fixes

arch/x86/kernel/early_printk.c:404:13: warning: incorrect type in assignment (different base types)
arch/x86/kernel/early_printk.c:404:13:    expected restricted __le16 [assigned] [usertype] wValue
arch/x86/kernel/early_printk.c:404:13:    got int [signed] value
arch/x86/kernel/early_printk.c:405:13: warning: incorrect type in assignment (different base types)
arch/x86/kernel/early_printk.c:405:13:    expected restricted __le16 [assigned] [usertype] wIndex
arch/x86/kernel/early_printk.c:405:13:    got int [signed] index
arch/x86/kernel/early_printk.c:406:14: warning: incorrect type in assignment (different base types)
arch/x86/kernel/early_printk.c:406:14:    expected restricted __le16 [assigned] [usertype] wLength
arch/x86/kernel/early_printk.c:406:14:    got int [signed] size
arch/x86/kernel/early_printk.c:845:16: warning: Using plain integer as NULL pointer
arch/x86/kernel/early_printk.c:992:13: warning: symbol 'enable_debug_console' was not declared. Should it be static?

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>