pci: Assign a default value on reads on error

pci: Assign a default value on reads on error

Many callers of PCI read functions do not check the return value for
error before using the variable that should contain the value read were
there not to be an error. However in the error case this variable is
never assigned to and so will contain uninitialised data.
To provide some certainty as to behaviour in the error case assign a
default value of all bits set.

This issue found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>