mmc: fsl_esdhc: Avoid infinite loop in esdhc_send_cmd_common()

The following hang is observed on a Hummingboard 2 MicroSOM
i2eX iMX6D - rev 1.3 with no eMMC populated on board:

U-Boot SPL 2018.11+gf6206f8587 (Nov 16 2018 - 00:56:34 +0000)
Trying to boot from MMC1

U-Boot 2018.11+gf6206f8587 (Nov 16 2018 - 00:56:34 +0000)

CPU:   Freescale i.MX6D rev1.5 996 MHz (running at 792 MHz)
CPU:   Extended Commercial temperature grade (-20C to 105C) at 33C
Reset cause: POR
Board: MX6 HummingBoard2
DRAM:  1 GiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1
Loading Environment from MMC... *** Warning - bad CRC, using default environment

No panel detected: default to HDMI
Display: HDMI (1024x768)
In:    serial
Out:   serial
Err:   serial
---> hangs

which is caused by the following infinite loop inside esdhc_send_cmd_common()

	while (!(esdhc_read32(&regs->irqstat) & flags))
		;

Instead of looping forever, provide an exit path so that a timeout
error can be propagated in the case irqstat does not report
any interrupts, which may happen when no eMMC is populated on
board.

Reported-by: Ricardo Salveti <rsalveti@rsalveti.net>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Ricardo Salveti <rsalveti@rsalveti.net>