spi: spi-mem: Check if exec_op function is set before calling it

Add check if exec_op is set before calling it.
At the moment it is called unconditionally, which leads to a crash if it
is not set correctly.

Signed-off-by: Bernhard Messerklinger <bernhard.messerklinger@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>