i2c: taos-evm: Make structure tsl2550_info constant

Static structure tsl2550_info, of type i2c_board_info, is referenced
only twice: the first time in arguments to dev_info() (which does not
modify it) and the second time as the last argument to function
i2c_new_device() (where the corresponding parameter is declared as
const). As tsl2550_info is therefore never modified, make it const to
protect it from unintended modifications.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>