Input: gscps2 - advance receive buffer write index

Commit 44f920069911 ("Input: gscps2 - use guard notation when
acquiring spinlock") moved the receive loop into gscps2_read_data()
and gscps2_report_data().

While moving the code, it preserved the writes to
buffer[ps2port->append], but omitted the following producer index
update from the original loop:

	ps2port->append = (ps2port->append + 1) & BUFFER_SIZE;

As a result, append never advances. Since gscps2_report_data() only
reports bytes while act != append, the receive buffer always appears
empty and no keyboard or mouse data reaches the serio core.

Restore the omitted index update.

Fixes: 44f920069911 ("Input: gscps2 - use guard notation when acquiring spinlock")
Cc: stable@vger.kernel.org # 6.13+
Signed-off-by: Xu Rao <raoxu@uniontech.com>
Link: https://patch.msgid.link/460B5655BA580C60+20260624094739.850306-1-raoxu@uniontech.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>