tty: hvsi: remove an extra variable from hvsi_write()

tty: hvsi: remove an extra variable from hvsi_write()

'source' is the same as 'buf'. Rename the parameter ('buf') to
'source' and drop the local variable.

Likely, the two were introduced to have a different type. But 'char' and
'unsigned char' are the same in the kernel for a long time.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230731080244.2698-4-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>