spi: ich: Limit slave->max_read_size

spi: ich: Limit slave->max_read_size

Since commit c4cb3b41f555 ("spi: ich: Correct max-size bug in ich_spi_adjust_size()")
(in v2020.04-rc1), SPI flash read no longer works with ICH SPI controller
in software sequencer mode.

ICH controller can only transfer a small number of bytes at once.
Before commit c4cb3b41f555, the logic happens to make sure data.nbytes
is limited to slave->max_write_size but after commit c4cb3b41f555
data.nbytes is no longer limited because slave->max_read_size is not
initialized with a valid number.

Fixes: c4cb3b41f555 ("spi: ich: Correct max-size bug in ich_spi_adjust_size()")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>