brcmfmac: Remove repeated calls to brcmf_chip_get_core()

There is no need to repeatdly call brcmf_chip_get_core(), which
traverses a list of cores every time its called (including during
register access code!).

Call it once, and store a pointer to the core structure. The existing
code does nto keep track of users of the cores anyway, and even so, this
will allow for easier refcounting in future.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>