thunderbolt: dma_port: Check 'dma_port_flash_write_block()'s return value

... and take the error path if it fails.

Fixes the following W=1 kernel build warning(s):

 drivers/thunderbolt/dma_port.c: In function ‘dma_port_flash_write_block’:
 drivers/thunderbolt/dma_port.c:331:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

Cc: Andreas Noever <andreas.noever@gmail.com>
Cc: Michael Jamet <michael.jamet@intel.com>
Cc: Yehezkel Bernat <YehezkelShB@gmail.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>