scsi: qedf: Add pointer checks in qedf_update_link_speed()

In the Linux kernel, the following vulnerability has been resolved:

scsi: qedf: Add pointer checks in qedf_update_link_speed()

The following trace was observed:

[ 14.042059] Call Trace:
[ 14.042061] <IRQ>
[ 14.042068] qedf_link_update+0x144/0x1f0 [qedf]
[ 14.042117] qed_link_update+0x5c/0x80 [qed]
[ 14.042135] qed_mcp_handle_link_change+0x2d2/0x410 [qed]
[ 14.042155] ? qed_set_ptt+0x70/0x80 [qed]
[ 14.042170] ? qed_set_ptt+0x70/0x80 [qed]
[ 14.042186] ? qed_rd+0x13/0x40 [qed]
[ 14.042205] qed_mcp_handle_events+0x437/0x690 [qed]
[ 14.042221] ? qed_set_ptt+0x70/0x80 [qed]
[ 14.042239] qed_int_sp_dpc+0x3a6/0x3e0 [qed]
[ 14.042245] tasklet_action_common.isra.14+0x5a/0x100
[ 14.042250] __do_softirq+0xe4/0x2f8
[ 14.042253] irq_exit+0xf7/0x100
[ 14.042255] do_IRQ+0x7f/0xd0
[ 14.042257] common_interrupt+0xf/0xf
[ 14.042259] </IRQ>

API qedf_link_update() is getting called from QED but by that time
shost_data is not initialised. This results in a NULL pointer dereference
when we try to dereference shost_data while updating supported_speeds.

Add a NULL pointer check before dereferencing shost_data.