lantiq: 5.15: fix compilation warning pciex fixup patch

lantiq: 5.15: fix compilation warning pciex fixup patch

pcibios_map_irq request a const pci_dev while pci_find_capability
doesn't. Cast dropping the const to fix compilation warning.
Fix compilation warning:
arch/mips/pci/fixup-lantiq.c: In function 'pcibios_map_irq':
arch/mips/pci/fixup-lantiq.c:34:33: error: passing argument 1 of 'pci_find_capability' discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
   34 |         if (pci_find_capability(dev, PCI_CAP_ID_EXP))
      |                                 ^~~
In file included from arch/mips/pci/fixup-lantiq.c:9:
./include/linux/pci.h:1129:40: note: expected 'struct pci_dev *' but argument is of type 'const struct pci_dev *'
 1129 | u8 pci_find_capability(struct pci_dev *dev, int cap);
      |                        ~~~~~~~~~~~~~~~~^~~
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>