net: hamradio: 6pack: fix uninit-value in sixpack_receive_buf

In the Linux kernel, the following vulnerability has been resolved:

net: hamradio: 6pack: fix uninit-value in sixpack_receive_buf

sixpack_receive_buf() does not properly skip bytes with TTY error flags.

The while loop iterates through the flags buffer but never advances the

data pointer (cp), and passes the original count (including error bytes)

to sixpack_decode(). This causes sixpack_decode() to process bytes that

should have been skipped due to TTY errors. The TTY layer does not

guarantee that cp[i] holds a meaningful value when fp[i] is set, so

passing those positions to sixpack_decode() results in KMSAN reporting

an uninit-value read.

Fix this by processing bytes one at a time, advancing cp on each

iteration, and only passing valid (non-error) bytes to sixpack_decode().

This matches the pattern used by slip_receive_buf() and

mkiss_receive_buf() for the same purpose.