brcmfmac: fix sparse error 'bad constant expression'

The definition of BRCMF_EVENTING_MASK_LEN results in a sparse
error message

.../fweh.c:331:22: error: bad constant expression
.../fweh.c:388:22: error: bad constant expression
.../dhd_common.c:256:22: error: bad constant expression

This is caused by the use of roundup() in BRCMF_EVENTING_MASK_LEN
and it is used to allocate an array variable on the stack. Better
use DIV_ROUND_UP() macro.

Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@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>