spi: dw: Round of n_bytes to power of 2

n_bytes variable in the driver represents the number of bytes per word
that needs to be sent/copied to fifo. Bits/word can be between 8 and 32
bits from the client but in memory they are a power of 2, same is mentioned
in spi.h header:
"
 * @bits_per_word: Data transfers involve one or more words; word sizes
 *      like eight or 12 bits are common.  In-memory wordsizes are
 *      powers of two bytes (e.g. 20 bit samples use 32 bits).
 *      This may be changed by the device's driver, or left at the
 *      default (0) indicating protocol words are eight bit bytes.
 *      The spi_transfer.bits_per_word can override this for each transfer.
"

Hence, round of n_bytes to a power of 2 to avoid values like 3 which
would generate unalligned/odd accesses to memory/fifo.

* tested on Baikal-T1 based system with DW SPI-looped back interface
transferring a chunk of data with DFS:8,12,16.

Fixes: a51acc24 ("spi: dw: Add support for 32-bits max xfer size")
Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Joy Chakraborty <joychakr@google.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20230512104746.1797865-4-joychakr@google.com


Signed-off-by: Mark Brown <broonie@kernel.org>