leds: flash: leds-qcom-flash: Declare the driver as a module

Explain in Kconfig that the driver can be compiled as a module.

Signed-off-by: Fenglin Wu <quic_fenglinw@quicinc.com>
Link: https://lore.kernel.org/r/20230725-leds-qcom-flash-driver-tiny-fixes-v2-1-0f5cbce5fed0@quicinc.com


Signed-off-by: Lee Jones <lee@kernel.org>