media: drxd_hard: better handle I2C errors

As warned by smatch:
	drivers/media/dvb-frontends/drxd_hard.c:989 HI_Command() error: uninitialized symbol 'waitCmd'.
	drivers/media/dvb-frontends/drxd_hard.c:1306 SC_WaitForReady() error: uninitialized symbol 'curCmd'.
	drivers/media/dvb-frontends/drxd_hard.c:1322 SC_SendCommand() error: uninitialized symbol 'errCode'.
	drivers/media/dvb-frontends/drxd_hard.c:1339 SC_ProcStartCommand() error: uninitialized symbol 'scExec'.

The error handling on several places are somewhat flawed, as
they don't check if Read16() returns an error.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>