leds: qti-flash: Replace usleep_range with mdelay

There is a path in the client driver which enables flash
in atomic context. Replace usleep_range() with mdelay() to
avoid a schedule_in_atomic_context failure.

Change-Id: I9ac8e235eb9a12e92ec9ac1fddd6b1452a01ba39
Signed-off-by: Kavya Nunna <knunna@codeaurora.org>