wb_spi incorrect number of bits transmitted when toggling `CHAR_LEN` config

When alternating CHAR_LEN configuration parameter [bits 6:0 of the CTRL (0x10) register], the third attempt fails.

Steps to reproduce:

1. Start a new SPI transaction with 0x28 as the LSB of the CTRL register (i.e. 40 bits to be transmitted). Verify that indeed 40 bits are transmitted.
2. The same as (1) but with 0x23 (i.e. 35 chars). Verify it still works.
3. The same as (1), back to 0x28. --BUG-- 0x29 gets written instead and 41 bits are transmitted.

Alternatively, just run the attached test bench for ~10us to see an assert fired when the bug occurs:

• wb_spi_tb.vhd
• Manifest.py