brcmsmac: split up wlc_phy_workarounds_nphy

The stack consumption in this driver is still relatively high, with one
remaining warning if the warning level is lowered to 1536 bytes:

drivers/net/wireless/broadcom/brcm80211/brcmsmac/phy/phy_n.c:17135:1: error: the frame size of 1880 bytes is larger than 1536 bytes [-Werror=frame-larger-than=]

The affected function is actually a collection of three separate implementations,
and each of them is fairly large by itself. Splitting them up is done easily
and improves readability at the same time.

I'm leaving the original indentation to make the review easier.

Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>