serial: 8250_uniphier: fix clearing divisor latch access bit

At this point, 'value' is always a byte, then this code is clearing
bit 15, which is already clear.  I meant to clear bit 7.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>