Fix serial.c for mach-omap1

An earlier serial.c patch broke all OMAP1 boards. NULL terminator entry for omap1
serial_platform_data cannot be removed just because serial driver uses
is as last entry marker. Without it we end this way:
Unable to handle kernel NULL pointer dereference at virtual address 00000028
pgd = c0004000
[00000028] *pgd=00000000
Internal error: Oops: 80000005 [#1]
last sysfs file:
Modules linked in:
CPU: 0    Not tainted  (2.6.32-rc5-06314-g4155da6-dirty #12)
PC is at 0x28
LR is at serial8250_config_port+0x184/0xc34
(...etc...)

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>