sfc: Const-qualify static data as appropriate, partly prompted by checkpatch

Fix the following warnings:

WARNING: struct dev_pm_ops should normally be const
WARNING: static const char * array should probably be static const char * const

Similarly const-qualify struct i2c_board_info, struct i2c_algo_bit_data,
struct efx_ethtool_stat, struct efx_mtd_ops and struct siena_nvram_type_info.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>