memstick: use sector_div instead of do_div

do_div is the wrong way to divide a sector_t, as it is less efficient
when sector_t is 32-bit wide.  With the upcoming do_div optimizations,
the kernel starts warning about this:

  drivers/memstick/core/ms_block.c: In function 'msb_io_work':
  include/asm-generic/div64.h:207:28: warning: comparison of distinct pointer types lacks a cast

This changes the code to use sector_div instead, which always produces
optimal code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>