drivers: uc81xx: Make busy wait implicit in most cases

Some commands need to signal that the chip is busy using the busy
pin. We generally need to wait for the chip to exit the busy state
before issuing a new command. Call uc81xx_busy_wait() from
uc81xx_write_cmd() just before issuing a new command instead to avoid
sprinkling wait calls everywhere.

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