Fix warning
uart.c:146:3: warning: ignoring return value of 'read', declared with attribute warn_unused_result [-Wunused-result]
Also reported by Coverity:
CID undefined (#1 of 1): Ignoring number of bytes read (CHECKED_RETURN)
check_return: "read(int, void *, size_t)" returns the number of bytes read, but it is ignored.