serial: ifx6x60: del_timer_sync must not be called in interrupt context.

This patch make use of del_timer instead of del_timer_sync in the
interrupt context.
The spi_timer function don't use any resources that may release after
running del_timer,
so using the del_timer is also safe and enough in this context.

Signed-off-by: Chen Jun <jun.d.chen@intel.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Tested-by: Chuansheng Liu <chuansheng.liu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>