brcmsmac: change pa_gain for bcm4313 iPA

The function wlc_lcnphy_load_tx_gain_table() has a target PA
gain specified for the iPA variant of the bcm4313. This gain
value is reduced to avoid PA distortion. The if-statement is
removed because it was rather redundant in the first place.
Please note that this patch does not provide full iPA support.

Cc: Jonas Gorski <jogo@openwrt.org>
Tested-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>