Minor correction in LPC USB support code.

Minor correction in LPC USB support code.

The USB_CMD_SEL_EP_CLRI command expects following data read,
but stall condition (USBC_EP_STAT_ST ) setting is achieved by
data write (the function is invoked lpc_write_cmd_data).
The right command label is USB_CMD_SET_EP_STAT, but it has
same value as USB_CMD_SEL_EP_CLR and is distinguished distinguishes
by following read only. So change has no real effect on compiled code.

The other problem is in function lpc_usb_read_endpoint()
if buffer size is smaller then selected maximal packet size.
Behavior corrected, but again no real effect for correct uses
of the function there.