wifi: brcmsmac: phy: Remove unreachable code

wlc_phy_txpwr_srom_read_nphy() in wlc_phy_attach_nphy() can not
return false, so it's impossible to get true value in this
if-statement. Also change those functions return types to void
since no one using it.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Artem Chernyshev <artem.chernyshev@red-soft.ru>
Reviewed-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20230814093621.289754-1-artem.chernyshev@red-soft.ru