serial: ifx6x60: changed internal bpw from boolean to int

driver should support 32bit SPI transfers. The boolean variable
only allowed 8/16.

Changed to support 8/16/32 for future enabling
of 32 bpw.

Signed-off-by: Russ Gorby <russ.gorby@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>