brcmfmac: rename variable max_seg_sz to max_seg_cnt for clarity

The variable max_seg_sz in brcmf_sdio_buffrw() respresents the maximum
number of buffers that can be sent in one MMC transfer request. Rename
it to max_seg_cnt to avoid confusion.

Reviewed-by: Franky 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>