serial: ifx6x60: fix paging fault on spi_register_driver

[  117.240866] BUG: unable to handle kernel paging request at 815b627c
[  117.240866] IP: [<813fe94b>] spi_register_driver+0xb/0x50
...
[  117.240866] Call Trace:
[  117.240866]  [<817de977>] ifx_spi_init+0xbe/0xf0

The root cause is, spi_register_driver() is trying to write into the
passed *const* struct spi_driver.

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>