qtnfmac: fix potential Spectre vulnerabilities

Fix potential Spectre vulnerabilities and other warnings
reported by smatch:

drivers/net/wireless/quantenna/qtnfmac/core.c:49 qtnf_core_get_mac() warn: potential spectre issue 'bus->mac' [r] (local cap)
drivers/net/wireless/quantenna/qtnfmac/core.c:51 qtnf_core_get_mac() warn: possible spectre second half.  'mac'
drivers/net/wireless/quantenna/qtnfmac/event.c:671 qtnf_event_parse() warn: potential spectre issue 'mac->iflist' [r] (local cap)
drivers/net/wireless/quantenna/qtnfmac/pcie/pearl_pcie.c:912 qtnf_pcie_skb_send() warn: variable dereferenced before check 'skb' (see line 881)

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>