The status register 1 (S1) is not writeable, hence we should not
write it. In order to clear the RDRF flag we only need to read
the data register.
Also, when stressing U-Boot a lot with serial input, an overflow can
occur which asserts the S1_OR flag (while not asserting the S1_RDRF
flag). To clear this flag we again just need to read the data
register, hence add this flag to the abort conditions for the while
loop.
Insert a compiler barrier to make sure reading the data register
gets executed after reading the status register.
