staging: goldfish: use div64_s64 instead of do_div

The goldfish nand driver divides a signed 64-bit number (loff_t)
in multiple places using the do_div() function. This has always
been unreliable but now produces a compiler warning (since 4.5-rc1):

goldfish/goldfish_nand.c: In function 'goldfish_nand_erase':
goldfish/goldfish_nand.c:107:91: error: comparison of distinct pointer types lacks a cast [-Werror]
goldfish/goldfish_nand.c: In function 'goldfish_nand_read_oob':
goldfish/goldfish_nand.c:145:91: error: comparison of distinct pointer types lacks a cast [-Werror]

This changes the code to the equivalent div_s64{,_rem} that
works correctly for negative numbers (which we should never
get here).

The warning has shown up on ARM allmodconfig builds after the goldfish
bus driver has become visible on ARM.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: bd2f348d ("goldfish: refactor goldfish platform configs")
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>