Reset bit count upon entering RXDATA

The but count should be set to 7 when entering RXDATA. previously it was
not reset or left at 0, which caused the first byte to only have the lsb
set and all other bits to be read incorrectly.

hw/fpga/sd_controller.sv

@@ -136,6 +136,7 @@ always_comb begin
             end
 
             if (~i_sd_data) begin
+                next_state.d_bit_count = '1;
                 next_state.macro = RXDATA;
             end