spi: bcm-qspi: fix calculation of address length

During implementation of the new API bcm_qspi_bspi_set_flex_mode() has
been modified breaking calculation of address length. An unnecessary
multiplication was added breaking flash reads.

Fixes: 5f195ee7 ("spi: bcm-qspi: Implement the spi_mem interface")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org