brcmfmac: drop duplicated core selection from brcmf_pcie_attach

It was left after reworking PCIe reset in commit 07fe2e38
("brcmfmac: Reset PCIE devices after recognition.").

Cc: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>