rsxx: remove extraneous 'const' qualifier

The returned string from rsxx_card_state_to_str is 'const',
but the other qualifier doesn't change anything here except
causing a warning with 'clang -Wextra':

drivers/block/rsxx/core.c:393:21: warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]
static const char * const rsxx_card_state_to_str(unsigned int state)

Fixes: f3791203

 ("block: IBM RamSan 70/80 trivial changes.")
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210323215753.281668-1-arnd@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>