cdrom: Reset sector_size back it is not 2048.

In v2.4.0-test2pre2 mmc_ioctl_cdrom_read_data() was extended by issuing
a MODE_SELECT opcode to change the sector size and READ_10 to perform
the actual read if the READ_CD opcode is not support.
The sector size is never changed back to the previous value of 2048
bytes which is however denoted by the comment for version 3.09 of the
cdrom.c file.

Use cdrom_switch_blocksize() to change the sector size only if the
requested size deviates from 2048. Change it back to 2048 after the read
operation if a change was mode.

Link: https://lkml.kernel.org/r/20201204164803.ovwurzs3257em2rp@linutronix.de


Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>