greybus: uart: Reduce UART count from 255 to 16

Arbitrary number 255 is both not aligned and probably too big.
Move the UART count down to 16 which is still large but, more realistic.
8 may be too few for future testing setups, 16 should accomodate any.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>