spi: Fix off-by-one error in calculating cycles per data frame

Issue: Configuring the frame length to certain values causes incorrect
operation.

Symptoms: Certain frame lengths result in the master sending one extra
clock pulse.  The slave device may then become desynchronized.

Workaround: The following frame lengths are supported and can be used.
Do not use other frame lengths.
	* Serial mode: 0, 2, 4, 6, 8
	* Dual mode:   0, 1, 3, 5, 7, 8
	* Quad mode:   0, 1, 2, 3, 5, 6, 7, 8