tty/serial/8250: Make omap hardware workarounds local to 8250.h

This allows us to get rid of the ifdefs in 8250.c.

Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>