dmi_scan: proper buf type in dmi_present()

dmi_scan: proper buf type in dmi_present()

buf needs to be unsigned in dmi_present(), otherwise dmi_base calculation
doesn't work properly leading to a wraparound during dmi_ioremap:

[    0.000000] ------------[ cut here ]------------
[    0.000000] WARNING: at arch/x86/mm/ioremap.c:524 __early_ioremap+0x85/0x18a()
[    0.000000] Modules linked in:
[    0.000000] Pid: 0, comm: swapper Not tainted 3.9.0-rc2-next-20130312 #283
[    0.000000] Call Trace:
[    0.000000]  [<c1036992>] warn_slowpath_common+0x72/0xa0
[    0.000000]  [<c19cc176>] ? __early_ioremap+0x85/0x18a
[    0.000000]  [<c19cc176>] ? __early_ioremap+0x85/0x18a
[    0.000000]  [<c10369e2>] warn_slowpath_null+0x22/0x30
[    0.000000]  [<c19cc176>] __early_ioremap+0x85/0x18a
[    0.000000]  [<c19cc1ed>] ? __early_ioremap+0xfc/0x18a
[    0.000000]  [<c19cc412>] early_ioremap+0xd/0xf
[    0.000000]  [<c19fbba9>] dmi_present+0xf5/0x22a
[    0.000000]  [<c19fbd9d>] dmi_scan_machine+0xbf/0x11c
[    0.000000]  [<c19bd781>] setup_arch+0x4db/0xca0
[    0.000000]  [<c168105c>] ? printk+0x3d/0x3f
[    0.000000]  [<c19ba763>] start_kernel+0x7b/0x329
[    0.000000]  [<c19ba361>] i386_start_kernel+0x137/0x13a
[    0.000000] ---[ end trace 4eaa2a86a8e2da22 ]---
[    0.000000] DMI not present or invalid.

Introduced in "dmi_scan: refactor dmi_scan_machine(), {smbios,dmi}_present()"

Signed-off-by: Artem Savkov <artem.savkov@gmail.com>
Acked-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>