hw/sd/sdcard: Rename Write Protect Group variables

hw/sd/sdcard: Rename Write Protect Group variables

'wp_groups' holds a bitmap, rename it as 'wp_group_bmap'.
'wpgrps_size' is the bitmap size (in bits), rename it as
'wp_group_bits'.

Patch created mechanically using:

  $ sed -i -e s/wp_groups/wp_group_bmap/ \
           -e s/wpgrps_size/wp_group_bits/ hw/sd/sd.c

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210728181728.2012952-4-f4bug@amsat.org>
Reviewed-by: Alexander Bulekov <alxndr@bu.edu>