spi: spi-mem: Allow controller supporting mem_ops without exec_op

spi: spi-mem: Allow controller supporting mem_ops without exec_op

Currently exec_op is always required if controller driver provides
mem_ops. But some controller such as bcm63xx-hsspi may only need to
implement other operation like supports_op and use the default
execution operation. This patch removes this restriction.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Link: https://lore.kernel.org/r/20230209200246.141520-13-william.zhang@broadcom.com
Signed-off-by: Mark Brown <broonie@kernel.org>