MIPS: Fix loading of modules with unresolved weak symbols

Loading of modules with unresolved weak symbols fails on MIPS
since '88173507

'.

Modules: handle symbols that have a zero value

The module subsystem cannot handle symbols that are zero.  If symbols
are present that have a zero value then the module resolver prints out a
message that these symbols are unresolved.

We have to use IS_ERR_VALUE() to check that a symbol has been resolved
or not.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>