serial:ifx6x60: Remove memset for SPI frame

There is no need to memset 0 to SPI frame memory before preparing
transfer frame bits, because SPI frame header are encoded with valid
data size, so don't need to worry about adopting dirty bits, more,
memset zero for each SPI frame may impact the spi throughput efficiency.

Signed-off-by: Chen Jun <jun.d.chen@intel.com>
Signed-off-by: channing <chao.bi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>