xen/serial: Return actual bytes stored in TX FIFO for OMAP

xen/serial: Return actual bytes stored in TX FIFO for OMAP

This is intended to decrease a time spending in transmitter
while waiting for the free space in TX FIFO.
And as result to reduce the impact of hvc on the entire system
running on OMAP5/DRA7XX based platforms.

Signed-off-by: Oleksandr Tyshchenko <oleksandr.tyshchenko@globallogic.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Julien Grall <julien.grall@citrix.com>
CC: Stefano Stabellini <stefano.stabellini@citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>