leds: lm355x: avoid enum conversion warning

clang points out that doing arithmetic between diffent enums is usually
a mistake:

drivers/leds/leds-lm355x.c:167:28: warning: bitwise operation between different enumeration types ('enum lm355x_tx2' and 'enum lm355x_ntc') [-Wenum-enum-conversion]
                reg_val = pdata->pin_tx2 | pdata->ntc_pin;
                          ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
drivers/leds/leds-lm355x.c:178:28: warning: bitwise operation between different enumeration types ('enum lm355x_tx2' and 'enum lm355x_ntc') [-Wenum-enum-conversion]
                reg_val = pdata->pin_tx2 | pdata->ntc_pin | pdata->pass_mode;
                          ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~

In this driver, it is intentional, so add a cast to hide the false-positive
warning. It appears to be the only instance of this warning at the moment.

Fixes: b98d13c7 ("leds: Add new LED driver for lm355x chips")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pavel Machek <pavel@ucw.cz>