brcmfmac: use debugfs_create_devm_seqfile() helper function

Some time ago the function debugfs_create_devm_seqfile() was
introduced in debugfs. The caller simply needs to provide a
device pointer and read function. The function brcmf_debugfs_add_entry()
is now simply a wrapper only doing the work for CONFIG_BRCMDBG.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>