sf: Avoid allocating memory on every read operation

At present spi_flash_cmd_read_ops() allocates and frees a few bytes of
memory every time it is called. It is faster to use the stack for this
and this is now supported by the minimum GCC version required by U-Boot.

Remove the allocation and use a variable-sized array instead.

Signed-off-by: Simon Glass <sjg@chromium.org>