spi: spi-ti-qspi: Handle truncated frames properly

We clamp frame_len_words to a maximum of 4096, but do not actually
limit the number of words written or read through the DATA registers
or the length added to spi_message::actual_length.  This results in
silent data corruption for commands longer than this maximum.

Recalculate the length of each transfer, taking frame_len_words into
account.  Use this length in qspi_{read,write}_msg(), and to increment
spi_message::actual_length.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org