memstick: avoid out-of-range warning

clang-14 complains about a sanity check that always passes when the
page size is 64KB or larger:

drivers/memstick/core/ms_block.c:1739:21: error: result of comparison of constant 65536 with expression of type 'unsigned short' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
        if (msb->page_size > PAGE_SIZE) {
            ~~~~~~~~~~~~~~ ^ ~~~~~~~~~

This is fine, it will still work on all architectures, so just shut
up that warning with a cast.

Fixes: 0ab30494

 ("memstick: add support for legacy memorysticks")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210927094520.696665-1-arnd@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>