pnv_phb4.c: check if root port exists in rc_config functions

pnv_phb4_rc_config_read() and pnv_phb4_rc_config_write() are asserting
the existence of the root port. The root port is now optional, and there
will be cases where a pnv-phb4 device won't have a root port attached.

Instead of asserting, check if the root port exists before read/writing
into it.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220105212338.49899-6-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>