usb: u_data_bridge: Return error when no USB TX request available

In some cases, it has been observed that all USB TX requests are
queued with USB controller and port bridge is trying to write data.
gbridge_port_write() API is returning zero (success of data) instead
of error code which results into data loss. Hence return -EAGAIN
when there is no USB TX request available. Also mark this error as
non-fatal by using pr_debug() here.

Change-Id: Id56788b372b62a005448a729e5f00de6fb895e47
Signed-off-by: Mayank Rana <mrana@codeaurora.org>