lm3554: simplify lm3554_s_flash_mode()

lm3554: simplify lm3554_s_flash_mode()

BZ: 32196

Accordingly to the datasheet there is no difference between torch and flash
registers when we set up the operating mode.

Change-Id: I5cbb7bd3c3ed5e2c6cf8713830dfebb2480dbdbb
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-on: http://android.intel.com:8080/43706
Reviewed-by: Shevchenko, Andriy <andriy.shevchenko@intel.com>
Reviewed-by: Toivonen, Tuukka <tuukka.toivonen@intel.com>
Reviewed-by: Cohen, David A <david.a.cohen@intel.com>
Tested-by: Koski, Anttu <anttu.koski@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>