serial: 8250_uniphier: avoid locking for FCR register write

The hardware book says, the FCR is combined with a register called
CHAR (it will trigger interrupt when a specific character is
received).  At first, I used lock/read/modify/write/unlock dance for
the FCR to not affect the upper bits, but the CHAR is actually never
used.  It should not hurt to always clear the CHAR and to handle the
FCR as a normal case.  It can save the costly locking.

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