wifi: wilc1000: set wilc_set_mac_address parameter as const

Any attempt to provide a const mac address to wilc_set_mac_address results
in the following warning:

warning: passing argument 2 of 'wilc_set_mac_address' discards 'const'
qualifier from pointer target type [-Wdiscarded-qualifiers]
[...]
drivers/net/wireless/microchip/wilc1000/hif.h:170:52: note: expected 'u8 *'
{aka 'unsigned char *'} but argument is of type 'const unsigned char *'a
int wilc_set_mac_address(struct wilc_vif *vif, u8 *mac_addr);

Instead of using an explicit cast each time we need provide a MAC address,
set the function parameter as const

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240514-mac_addr_at_probe-v2-3-afef09f1cd10@bootlin.com