lp8788-charger: Use PAGE_SIZE for the sysfs read operation

The sysfs allocates PAGE_SIZE. It is used by each R/W operation method.
Use it instead of another buffer size.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>