Removes rate limiting from kupload.sh

Commit 7123bcb0fcca9e39c35f4b67088b581cdf2fb50d changed the UART driver
to work around ZMODEM uploads waiting forever on the next rx_watermark
IRQ that would never come.

With this workaround, we no longer need to rate limit ZMODEM transfers
on the sender side.

Change-Id: I693dd3bca98f446739a1f01ba537543ffb5dd03e