platform/chrome: cros_ec_spi: drop unneeded BUG_ON()

In the context, the following conditions are always false:

- `todo` < 0
Suppose that EC_SPI_FRAME_START is found at the last byte of transfer.

In the case, `ptr` == `end` - 1.  As a result, `todo` must be 0.

- `todo` > `ec_dev->din_size`
Suppose that there is no preamble bytes.  EC_SPI_FRAME_START is found at
the first byte of transfer.

In the case, `end` == `ptr` + EC_MSG_PREAMBLE_COUNT.
As a result, `todo` == EC_MSG_PREAMBLE_COUNT - 1.
However, it already checked `ec_dev->din_size` < EC_MSG_PREAMBLE_COUNT at
the beginning of function.

Drop the unneeded BUG_ON().

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220513044143.1045728-7-tzungbi@kernel.org