spi: fix mxs_spi_slave structure allocation to clear memory

spi: fix mxs_spi_slave structure allocation to clear memory

Use calloc() instead of malloc() to allocate the mxs_spi_slave structure.
Clearing the memory is necessary since most of the time this gets done
super early in boot, but on warm reboots, and when SPI probing is done
long after the init stages it could actually pick up previously used memory,
and things like the chipselect polarity and other data end up being filled
with trash data if not explicitly set by the board files.

This solves a semi-random, almost unreproducable error whereby SPI devices
act very, very strangly on boot.

Signed-off-by: Matt Sealey <matt@genesi-usa.com>
Acked-by: Stefano Babic <sbabic@denx.de>