brcmfmac: rework debugfs functions in the driver

Reworked the debugfs functions in the driver making it easier for
other driver parts to add a debugfs entry and keeping the information
they want to expose in debugfs private, ie. not in a header.

This is accomplished by providing the function brcmf_debugfs_add_entry()
in which the caller provides a read function in which they provide the
content. The debugfs function will take care of creating the debugfs
entry and cleaning up upon removal.

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: John W. Linville <linville@tuxdriver.com>