cmd/misc: Stop using a function pointer

cmd/misc: Stop using a function pointer

Currently, enabling CMD_MISC gives:
cmd/misc.c:67:25: warning: assignment to 'int (*)(struct udevice *, int,  void *, int)' from incompatible pointer type 'int (*)(struct udevice *, int,  const void *, int)' [-Wincompatible-pointer-types]

Because 'misc_read' takes a void * and 'misc_write' takes a const void
*, both of which make sense for their operation.  Given there's one
place we make use of the function pointer, just call read or write
directly for the operation we're called with.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>