brcmfmac: Do not use strcpy and strcat

Commit "c1b20532 brcmfmac: Make firmware path a module parameter"
introduced use of strcpy and strcat. The strcpy and strcat require
using null terminated strings and can cause out-of-bounds memory
access and subsequent corruption. This patch replaces these by
strncpy and strncat respectively to assure array boundaries are
not crossed.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Daniel Kim <dekim@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>