[MTD] mtdchar.c silence sparse warning

The copy_to_user was casting away the address space to get the offset of
the length member.  Use offsetof() instead and add it to the void __user
*argp.

drivers/mtd/mtdchar.c:527:23: warning: cast removes address space of expression
drivers/mtd/mtdchar.c:527:23: warning: incorrect type in argument 1 (different address spaces)
drivers/mtd/mtdchar.c:527:23:    expected void [noderef] <asn:1>*to
drivers/mtd/mtdchar.c:527:23:    got unsigned int *<noident>

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>