[MTD] m25p80 converted to mutex

Convert semaphore usage in m25p80 driver to mutex; mention another kind of
SPI flash chip that should be able to use this driver (given minor tweaks).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>