otg: Improve IN write performance

chunks_exact() can be handled by the compiler more efficiently.
Previous code was making a memcpy call for each 4 byte chunk slice.
Hoisting the fifo out of the loop avoids recalculating the pointer each
time.

In my benchmark I see a jump from ~13 megabyte/sec to ~25MB/sec after
this change (opt-level=3). opt-level = "z" goes 9MB/s to 18MB/s.

The benchmark was on a stm32h7s3l8, 600mhz clock, 512 byte bulk writes,
data in DTCM. The benchmark isn't just USB writes, also has some
unrelated memcpys for packet construction.

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