serial: serial_sh: bugfix: autoboot fails if serial console is not connected

On kzm9g board (rmobile SoC), autoboot fails if serial console cable is not
connected.  When serial cable is not connected, serial error occurs and
some garbage comes in data register.
sh_serial_tstc() in serial_sh.c does not check error status and misunderstand
there is some input data.  It is the reason that autoboot fails.
This patch adds checking error status in sh_serial_tstc().

This patch is based on v2013.01-rc1 tag of u-boot master git.

Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>