mpc52xx_spi: fix clearing status register

Before reading status register to check MODF failure, we have to clear it
first since the MODF flag will be set after initializing the spi master,
if the hardware comes up with a low SS. The processor datasheet reads:
Mode Fault flag -- bit sets if SS input goes low while SPI is configured as a
master. Flag is cleared automatically by an SPI status register read (with MODF
set) followed by a SPI control register 1 write.
Hence simply rereading the register is not sufficient to clear the flag. We
redo the write also to make sure to clear the flag.

V2 Changes:
* change variable type from int to u8

Signed-off-by: Luotao Fu <l.fu@pengutronix.de>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>