altos/avr: SPI mutex is now held by the caller, not the SPI driver

altos/avr: SPI mutex is now held by the caller, not the SPI driver

SPI transactions generally require a read followed by a write, with
the chip select held the whole time. As a result, the SPI bus must be
held across multiple transactions. To make this reliable, the caller
must hold the SPI mutex, instead of the underlying SPI driver.

Signed-off-by: Keith Packard <keithp@keithp.com>