Check for zero length in the legacy spi_flash_read() /
spi_flash_write() / spi_flash_erase() functions.
On zero length, return 0 immediately, don't call the underlying method.

Rationale:
- these legacy functions call the _read(), _write() and _erase() methods
  of struct mtd
- the DM callers of these methods already check for zero length
- making all callers of these methods check for zero length makes it
  possible to remove the check from implementations of these _read(),
  _write() and _erase() methods

Signed-off-by: Marek Behún <marek.behun@nic.cz>