extmod/machine_spi: Remove EVENT_POLL_HOOK from soft-SPI transfer func.

SPI needs to be fast, and calling the EVENT_POLL_HOOK every byte makes it
unusable for ports that need to do non-trivial work in the EVENT_POLL_HOOK
call.  And individual SPI transfers should be short enough in time that
EVENT_POLL_HOOK doesn't need to be called.

If something like this proves to be needed in practice then we will need
to introduce separate event hook macros, one for "slow" loops (eg
select/poll) and one for "fast" loops (eg software I2C, SPI).