bootm: fix size arg of flush_cache() in bootm_load_os().

Variable _load_end_ points to end address of uncompressed buffer
(*not* uncomress_buffer_end / sizeof(ulong)), so multipling uncompressed
size with sizeof(ulong) is grossly incorrect in flush_cache().
It might lead to access of address beyond valid memory range and hang the CPU.

Tested on MIPS architecture by using compressed(gzip, lzma)
and uncompressed uImage.

Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>