[ARM] i.MX serial: fix tx buffer overflows

Fix occasional tx buffer overflows in the i.MX serial driver which came
from the fact that space in the buffer was checked after sending the first
byte.  Also, fifosize is 32 bytes, not 8.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>