The FIFO_DEPTH hardware configuration parameter can be found from the
power-on value of RX_WMark in the FIFOTH register. This is used to
initialise the watermarks, but when calculating the number of free fifo
spaces a preprocessor definition is used which is hard coded to 32.

Fix reading the value out of FIFOTH (the default value in the RX_WMark
field is FIFO_DEPTH-1 not FIFO_DEPTH). Allow the fifo depth to be
overriden by platform data (since a bootloader may have changed FIFOTH
making auto-detection unreliable). Store the fifo_depth for later use.
Also fix the calculation to find the number of free bytes in the fifo to
include the fifo depth in the left shift by the data shift, since the
fifo depth is measured in fifo items not bytes.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>