leds: leds-qpnp: Added in delay for flash LED operation

Flash LEDs require VIN_CHG to reach 5V to fire stabely. Adding delay
between flash enable and assert strobe when flash LEDs are turned on
and between flash strobe desert and module disable provides an improved
proformance. This also simplifies enable sequence so that the current
module enable sequence 0x00 to 0x80 before going to 0xE0 un-necessary.

CRs-Fixed: 539632, 548259
Change-Id: I26472c9ba445f3a311814ede8a821fa580f460b8
Signed-off-by: Chun Zhang <chunz@codeaurora.org>