serial: ns16550: drop CONFIG_OMAP1610 from the special case

If CONFIG_OMAP1610 is defined, the code returning the fixed value (26)
is enabled. But this case is covered by the following code.

(CONFIG_SYS_NS16550_CLK + (gd->baudrate * (MODE_X_DIV / 2))) /
    (MODE_X_DIV * gd->baudrate)
= (48000000 + (115200 * (16 / 2))) / (16 * 115200)
= 48921600 / 1843200
= 26

The "#ifdef CONFIG_OMAP1610" was added by commit 6f21347d more than
ten years ago. In those days, the divide-and-round was not used.
I guess that is why this weird code was added here.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Rini <trini@ti.com>
Cc: Rishi Bhattacharya <rishi@ti.com>