serial: zynq: Use BIT macros instead of shifts and full hex numbers

Coding style is checking to use BIT macros instead of shifts.
The patch is also fixing the rest of macros which should be BITs instead
of hex numbers.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>