serial: fix retry logic

I'm not sure if the retry logic has ever worked when not using FIFO mode.  I
found this while writing a test case although code inspection confirms it is
definitely broken.

The TSR retry logic will never actually happen because it is guarded by an
'if (s->tsr_rety > 0)' but this is the only place that can ever make the
variable greater than zero.  That effectively makes the retry logic an 'if (0)'.

I believe this is a typo and the intention was >= 0.  Once this is fixed though,
I see double transmits with my test case.  This is because in the non FIFO
case, serial_xmit may get invoked while LSR.THRE is still high because the
character was processed but the retransmit timer was still active.

We can handle this by simply checking for LSR.THRE and returning early.  It's
possible that the FIFO paths also need some attention.

Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
