staging: wilc1000: use sizeof(variable) for memory allocated to store key info

Changes to fix below checkpatch reported issues.

CHECK: Prefer kmalloc(sizeof(*priv->wilc_gtk[idx])...) over
kmalloc(sizeof(struct wilc_wfi_key)...)

CHECK: Prefer kmalloc(sizeof(*priv->wilc_ptk[idx])...) over
kmalloc(sizeof(struct wilc_wfi_key)...)

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>