common/console.c: discard volatile

Avoid errors of like

common/console.c: In function ‘console_record_reset’:
common/console.c:615:16: error: passing argument 1 of ‘membuff_purge’
discards ‘volatile’ qualifier from pointer target type
[-Werror=discarded-qualifiers]
  615 |  membuff_purge(&gd->console_out);
      |                ^~~~~~~~~~~~~~~~

by casting to non-volatile.

The volatile property stems from declarations like

arch/arm/include/asm/global_data.h:114:

But there is no need to treat gd->console_out and gd->console_in as
volatile in the context of common/console.c.

Fixes: b6123128 ("console: Add a function to read a line of the output / eof")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>