drivers: ssd16xx: Refactor busy pin handling

The SSD16xx driver needs to wait for the chip to complete commands
before issuing new commands. The datasheet doesn't specify which
commands cause the chip to raise the busy pin. The current driver has
ssd16xx_busy_wait() calls after commands that are known to take a long
time. This is very fragile and scatters a lot of boiler plate around
the driver.

Include an ssd16xx_busy_wait() call in ssd16xx_write_cmd() instead of
after commands that are suspected to take a long time. This makes the
driver more robust and is more in line with the expectations set out
in the data sheet.

Signed-off-by: Andreas Sandberg <andreas@sandberg.uk>