i2c/tsl2550: Speed up initialization

There's some redundancy in the tsl2550 initialization sequence. It is
powering up the device twice, and setting the operating mode twice
too. Setting things just once saves SMBus transactions, which aren't
always cheap, speeding up the device initialization.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Rodolfo Giometti <giometti@linux.it>