shrink uca.ibufsize from UCHCOMIBUFSIZE to wMaxPacketSize of bulk-in pipe

shrink uca.ibufsize from UCHCOMIBUFSIZE to wMaxPacketSize of bulk-in pipe

USB transaction is finished when whole requested data has transferred,
or short packet (the size is less than wMaxPacketSize) has sent.

UCHCOMIBUFSIZE(256) was multiply of wMaxPacketSize(32). When CH340 sends
exact wMaxPacketSize byte packet, this will cause Rx jam problem
due to transaction is not finished.

Now uca.ibufsize is same as wMaxPacketSize to avoid this problem.

ok kevlo@