spi, spi_mxc: do not hang in spi_xchg_single

if status register do never set MXC_CSPICTRL_TC, spi_xchg_single
endless loops. Add a timeout here to prevent endless hang.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Dirk Behme <dirk.behme@gmail.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>