brcmfmac: More efficient and slightly easier to read fixup for 4339 chips

Its more efficient to test the register we're interested in first,
potentially avoiding two more comparisons, and therefore always avoiding
one comparison per call on all other chips.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
[arend: fix some checkpatch warnings]
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>