lp8727_charger: Small cleanup in naming

"pdata" and "pchg->pdata" are the same.  Changing the function call to
pdata->get_batt_present() makes it match the check and it's a little
cleaner.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>