spi/bcm63xx: fix typo in bcm63xx_spi_max_length breaking compilation

Fix compilation by renaming argument dev to spi as expected by the code.

Fixes the following error:

drivers/spi/spi-bcm63xx.c: In function ‘bcm63xx_spi_max_length’:
drivers/spi/spi-bcm63xx.c:434:50: error: ‘spi’ undeclared (first use in this function)
  struct bcm63xx_spi *bs = spi_master_get_devdata(spi->master);
                                                  ^~~

Fixes: 0135c03d ("spi/bcm63xx: make spi subsystem aware of message size limits")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>