brcmfmac: Fix off by one bug in brcmf_count_20mhz_channels()

In the brcmf_count_20mhz_channels function we are looping through a list
of channels received from firmware. Since the index of the first channel
is 0 the condition leads to an off by one bug. This is causing us to hit
the WARN_ON_ONCE(1) calls in the brcmu_d11n_decchspec function, which is
how I discovered the bug.

Introduced by:
commit b48d8916
("brcmfmac: rework wiphy structure setup")

Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>