This commit changes the transfer length register to work in increments of
64 bytes and without offset. The true transfer length can now be
determined by multiplying the value of the transfer_length register with
64.
A value of zero is interpreted as a request for all available storage.
Additionally, this commit fixes an off by one issue that was discovered
during testing of the RX path.
Signed-off-by: David Winter <david.winter@analog.com>