spi: bcm-qspi: Fix build failure caused by spi_flash_read() API removal

Patch http://patchwork.ozlabs.org/patch/905205/

 has been partially
applied, and changes to the bcm-qspi driver have been lost somehow
(probably due to a conflict when applying the patch).

Remove the ->spi_flash_read() bits from this driver to fix the build
error.

Fixes: c1f5ba70 ("spi: Get rid of the spi_flash_read() API")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>